cmd/go: fix non-script staleness checks interacting badly with GOFLAGS

Fixes #43012.

Change-Id: Idc7a64b53c411e6dadd98521a48e15e664737d42
GitHub-Last-Rev: b56c088
GitHub-Pull-Request: #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>

src/cmd/go/go_test.go

@@ -216,6 +216,7 @@ func TestMain(m *testing.M) {
 	}
 	// Don't let these environment variables confuse the test.
 	os.Setenv("GOENV", "off")
+	os.Unsetenv("GOFLAGS")
 	os.Unsetenv("GOBIN")
 	os.Unsetenv("GOPATH")
 	os.Unsetenv("GIT_ALLOW_PROTOCOL")