CL 168697 unconditionally set GOPATH in dist, which broke the
misc-vet-vetall builder, because cmd/vet/all depends on GOPATH.

Fixes #30971

Change-Id: If6a58e054c6a4fedc2ea506a2c443348489c91f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/169217
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>