golang: Prepare for future GOPATH removal

GOPATH is going to be removed in Go1.17 (see e.g. https://github.com/golang/go/issues/37755#issuecomment-771879911).

-> Prevent programs suddenly become installed into $HOME/go/bin instead of
gowork/bin, and mod cache to become something like $HOME/go/... instead of
being kept under gowork/

No change in behaviour for Go ≤ 1.16