cmd/go: fix non-script staleness checks interacting badly with GOFLAGS
Fixes #43012.
Change-Id: Idc7a64b53c411e6dadd98521a48e15e664737d42
GitHub-Last-Rev:
b56c0880c3b5ceb86c6fa2ba6bf82f8969e10472
GitHub-Pull-Request: golang/go#43155
Reviewed-on: https://go-review.googlesource.com/c/go/+/277453
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>