Adam Parkin’s answer won’t work because, at least on windows, the shell will not substitute environment variables given as arguments. ${env:...} variables as suggested in a comment on that answer won’t be substituted using environment variables set in tasks.json itself, only preexisting ones. You can however add custom settings in settings.json, and reference those in tasks.json using ${config:...}.

e.g. settings.json:

{
    "win_dir": "build_windows",
    "linux_dir": "build",
    "osx_dir": "build_osx",
    "configure": "../configure --with-bash-malloc=no CFLAGS="-O3 -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-free" LDFLAGS=-L/usr/local/lib LIBS="-ltcmalloc -lcurl" CC=clang"
}

in tasks.json:

{
    "tasks": [
        {
            "taskName": "configure",
            "command": "bash",
            "windows": {
                "args": ["-c", "rm -rf ${config:win_dir}; mkdir ${config:win_dir}; cd ${config:win_dir}; ${config:configure}"]
            },
            "linux": {
                "args": ["-c", "rm -rf ${config:linux_dir}; mkdir ${config:linux_dir}; cd ${config:linux_dir}; ${config:configure}"]
            },
            "osx": {
                "args": ["-c", "rm -rf ${config:osx_dir}; mkdir ${config:osx_dir}; cd ${config:osx_dir}; ${config:configure}"]
            },
            "isBuildCommand": true,
            "problemMatcher": "$make-compile"
        },
        // ... other tasks using the variables
    ]
}

Use tasks.options.env to set per-task environment

As documented in the VSCode custom tasks page, environment variables can be set either at the task level or the file level, depending on how broadly those values should be applied.

Task-level Example:

{
    "version": "2.0.0",
    "tasks": [
        {
            "label": "Test with passing variable",
            "type": "shell",
            "options": {
                "env": {
                    "GREETING_VAR": "Hello"
                }
            },
            "command": "echo ${GREETING_VAR}, World!"
        }
    ]
}

File-level Example:

{
    "version": "2.0.0",
    "options": {
         "env": {
              "GREETING_VAR": "Hello"
         }
    },
    "tasks": [
        {
            "label": "Test with passing variable",
            "type": "shell",
            "command": "echo ${GREETING_VAR}, World!"
        }
    ]
}

More details regarding options in tasks may be found here.

Thus the question is if it is possible to define your own variables, so you can use them similar to ${workspaceRoot}.

You could define environment variables in your tasks.json:

{
    // See https://go.microsoft.com/fwlink/?LinkId=733558
    // for the documentation about the tasks.json format
    "version": "2.0.0",
    "options": {
        "env": {
            "win_dir": "build_windows",
            "linux_dir": "build",
            "osx_dir": "build_osx",
            "configure": "../configure --with-bash-malloc=no CFLAGS="-O3 -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-free" LDFLAGS=-L/usr/local/lib LIBS="-ltcmalloc -lcurl" CC=clang"
        }
    },
    "tasks": [
        {
            "label": "Example",
            "type": "shell",
            "command": "echo win_dir is $env:win_dir"
        },
    ]
}

With that, you could then also use the environment matching to refer to the relevant environment variables.

I am taking a different approach.

tasks.json

{
    "version": "2.0.0",
    "params":{
        "git_version":"2.30.0",
        "node_version":"14.13.6",
        "python_version":"3.8"
    },
    "tasks": [
        {
            "label":"Process Task.json",
            "type":"shell",
            "command":"python process_tasks.py",
            "group":"build",
            "isBackground":true
        },
        {
            "label":"Test process_tasks.py",
            "type":"shell",
            "command":"echo $[params.git_version]",
            "group":"test",
            "presentation": {
                "reveal": "always"
            }
        }
     ]
}

Rather than making env variables, we can follow the flowing steps:

Step 1:

Make a task in tasks.json as follows

{
    "label":"Process Task.json",
    "type":"shell",
    "command":"python process_tasks.py",
    "group":"build",
    "isBackground":true
},

Step 2:

process_tasks.py is a Python file that will replace variables in tasks.json with the actual value:

import json
import os
import re

if __name__ == "__main__":
    # Get the path to the JSON file
    json_path = os.path.join(".vscode/tasks.json")

    with open(json_path, "r") as f:
        data = json.load(f)

    with open(json_path, "r") as f:
        lines = f.readlines()

        new_lines = []

        #regex to find text between $[]
        regex = re.compile(r"$[(.*?)]")

        for line in lines:
            #regex in line:
            match = regex.search(line)
            if match:
                #get the text between $[]

                text = match.group(1)

                keys = text.split(".")

                buffer_data = data

                for key in keys:
                    buffer_data = buffer_data[key]

                #replace the text with the value of the environment variable
                line = line.replace(f"$[{text}]", buffer_data)

            new_lines.append(line)

    with open(json_path, "w") as f:
        f.writelines(new_lines)

Step 3:

Add a test task to verify your result

{
    "label":"Test process_tasks.py",
    "type":"shell",
    "command":"echo $[params.git_version]",
    "group":"test",
    "presentation": {
        "reveal": "always"
    }
},

Note:

Making this “Process Task.json” as a global task and adding the correct path of the process.py file in the build task will reduce a lot of work.

Thus we can define our own variables inside tasks.json and access them using $[params.git_version].

After executing the “Process Task.json” task, all variables in $[] format will be replaced by its corresponding value.