compress/gzip: skip TestGZIPFilesHaveZeroMTimes on non-builders

Fixes #18604

Change-Id: I89221d5e632042167dfced068e1dc14e932cd618
Reviewed-on: https://go-review.googlesource.com/35111
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/gzip/issue14937_test.go

@@ -9,11 +9,17 @@ import (
 	"testing"
 )
 
-// Per golang.org/issue/14937, check that every .gz file
-// in the tree has a zero mtime.
+// TestGZIPFilesHaveZeroMTimes checks that every .gz file in the tree
+// has a zero MTIME. This is a requirement for the Debian maintainers
+// to be able to have deterministic packages.
+//
+// See https://golang.org/issue/14937.
 func TestGZIPFilesHaveZeroMTimes(t *testing.T) {
-	if testing.Short() && testenv.Builder() == "" {
-		t.Skip("skipping in short mode")
+	// To avoid spurious false positives due to untracked GZIP files that
+	// may be in the user's GOROOT (Issue 18604), we only run this test on
+	// the builders, which should have a clean checkout of the tree.
+	if testenv.Builder() == "" {
+		t.Skip("skipping test on non-builder")
 	}
 	goroot, err := filepath.EvalSymlinks(runtime.GOROOT())
 	if err != nil {