cmd/builder: pass through $GO_TEST_TIMEOUT_SCALE

Change-Id: I518010751a4d1b98dcec63a3bf8fdd08bf57fabe
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/9301
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/builder/env.go

@@ -297,6 +297,7 @@ func extraEnv() []string {
 		"PATH",
 		"TMPDIR",
 		"USER",
+		"GO_TEST_TIMEOUT_SCALE", // increase test timeout for slow builders
 	}
 	if runtime.GOOS == "plan9" {
 		extra = append(extra, "objtype", "cputype", "path")