cmd/go: fix TestBuildIDContainsArchModeEnv

Changing GOARCH, GOARM, GO386 leads to a stale dependency.

Updates #24436.

Change-Id: I5b5b3fca6401be50fa81fb040bc56356de7555de
Reviewed-on: https://go-review.googlesource.com/112975
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -4851,7 +4851,7 @@ func main() {}`)
 			before()
 			tg.run("install", "mycmd")
 			after()
-			tg.wantStale("mycmd", "stale dependency: runtime/internal/sys", "should be stale after environment variable change")
+			tg.wantStale("mycmd", "stale dependency", "should be stale after environment variable change")
 		}
 	}