Since GO111MODULE=on by default, the Go command needs a location for
the module cache, even though it doesn't need to be written when
building std and cmd. If GOROOT is checked out to $HOME/go, which is
also the default location for GOPATH, this causes unnecessary problems
late in the build.

With this change, dist sets GOPATH to $GOROOT/pkg/obj/go-path. This is
next to the temporary GOCACHE, $GOROOT/pkg/obj/go-build.

Fixes #30960

Change-Id: I60771ee7f7c67ced1d2dc7c66b5885703fad1b63
Reviewed-on: https://go-review.googlesource.com/c/go/+/168697
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>