I have a flake.nix like this:

{
  description = "EMR Serverless compatible Python env";

  # Flake inputs
  inputs = {
    nixpkgs.url = "https://flakehub.com/f/NixOS/nixpkgs/0.2405.*.tar.gz";
    nixpkgs-python.url = "github:cachix/nixpkgs-python";
  };

  # Flake outputs
  outputs = { self, nixpkgs, nixpkgs-python }:
    let
      # Systems supported
      allSystems = [
        "x86_64-linux" # 64-bit Intel/AMD Linux
        "aarch64-linux" # 64-bit ARM Linux
        "x86_64-darwin" # 64-bit Intel macOS
        "aarch64-darwin" # 64-bit ARM macOS
      ];

      # Helper to provide system-specific attributes
      forAllSystems = f: nixpkgs.lib.genAttrs allSystems (system: f {
        pkgs = import nixpkgs { inherit system; };
      });
    in
    {
      # Development environment output
      devShells = forAllSystems ({ pkgs }: {
        default =
          let
            python = nixpkgs-python.packages.x86_64-linux."3.9.16";
          in
          pkgs.mkShell {
            # The Nix packages provided in the environment
            packages = [
              # Python plus helper tools
              (python.withPackages (ps: with ps; [
                virtualenv # Virtualenv
                pip # The pip installer
              ]))
            ];
          };
      });
    };
}

The issue is that the Python system is fixed to x86_64-linux here:

python = nixpkgs-python.packages.x86_64-linux."3.9.16";

I think that it should be platform-specific, somehow using system.

How do I achieve this?