I intend to build go into /usr/local/go120 using go from /usr/local/go117

I unpack the archive (1.20) into /usr/local/go120

I cd into /usr/local/go120/src

I run all.bash and I get …

  Building Go cmd/dist using /usr/local/go117. (go1.17.13 freebsd/amd64)
  Building Go toolchain1 using /usr/local/go117.
  Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
  Building Go toolchain2 using go_bootstrap and Go toolchain1.
  go: cannot find GOROOT directory: /usr/local/go120

I have the following in in my zshrc

export GOROOT=/usr/local/go117
export PATH=${GOROOT}/bin:$PATH

What’s the magic sauce to make this work?