cmd: ensure that GOPATH is always valid in subprocesses that execute 'go build'

GOPATH/pkg contains, among other things, the module cache (and associated lockfiles).

Fixes #30776

Change-Id: I305cb3c0daab8cedd2e6ad235d4733f66af18723
Reviewed-on: https://go-review.googlesource.com/c/go/+/167082


Reviewed-by: Jay Conrod <jayconrod@google.com>