all: warn if user hasn't run gcloud auth application-default login

"gcloud auth application-default login" is NOT the same as
"gcloud auth login"

... which is super confusing.

Add a check for users.

We should probably add this in more places over time. Baby steps.

Change-Id: I4584f5b84b941fa6893eccd09fd61e8850538607
Reviewed-on: https://go-review.googlesource.com/130195
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cmd/release/release.go b/cmd/release/release.go
index 7b624c5..9dd48a4 100644
--- a/cmd/release/release.go
+++ b/cmd/release/release.go
@@ -60,6 +60,7 @@
 	rand.Seed(time.Now().UnixNano())
 
 	if *uploadMode {
+		buildenv.CheckUserCredentials()
 		if err := upload(flag.Args()); err != nil {
 			log.Fatal(err)
 		}