Add a warning message about missing gcert at the end of the build

Now that we are ready to rollout gcert, adding a warning that builds
will fail if gcert doesn't exist. Doing this only for builds with
ANDROID_BUILD_ENVIRONMENT_CONFIG=googler.

Test: Ran a build without gcert and USE_RBE=true and it printed the
warning.
Bug: b/229743321

Change-Id: I616a4a534ab277873ac7e19baecb78620f07854c

ui/build/rbe.go

@@ -119,6 +119,7 @@ func startRBE(ctx Context, config Config) {
 }
 
 func stopRBE(ctx Context, config Config) {
+	defer checkProdCreds(ctx, config)
 	cmd := Command(ctx, config, "stopRBE bootstrap", rbeCommand(ctx, config, bootstrapCmd), "-shutdown")
 	output, err := cmd.CombinedOutput()
 	if err != nil {
@@ -131,6 +132,15 @@ func stopRBE(ctx Context, config Config) {
 	}
 }
 
+func checkProdCreds(ctx Context, config Config) {
+	if !config.IsGooglerEnvironment() || config.GoogleProdCredsExist() {
+		return
+	}
+	fmt.Fprintln(ctx.Writer, "")
+	fmt.Fprintln(ctx.Writer, "\033[33mWARNING: Missing LOAS credentials, please run `gcert`. This will result in failing RBE builds in the future, see go/build-fast#authentication.\033[0m")
+	fmt.Fprintln(ctx.Writer, "")
+}
+
 // DumpRBEMetrics creates a metrics protobuf file containing RBE related metrics.
 // The protobuf file is created if RBE is enabled and the proxy service has
 // started. The proxy service is shutdown in order to dump the RBE metrics to the