Add google_prod_creds to the list of auth mechanisms

This is to ensure that we don't default to using
application_default_creds when it is not set and rather
"RBE_use_google_prod_creds" variable is set. This is necessary to
rollout RBE gcert auth.

Bug: b/224446621
Change-Id: I51e3f1aefadf282240b8f861efafde6b9b61d305
diff --git a/ui/build/config.go b/ui/build/config.go
index 01fe8fa..dd5bd0c 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -187,7 +187,7 @@
 	}
 
 	if err := fetchEnvConfig(ctx, config, bc); err != nil {
-		fmt.Fprintf(os.Stderr, "Failed to fetch config file: %v", err)
+		fmt.Fprintf(os.Stderr, "Failed to fetch config file: %v\n", err)
 	}
 
 	configDirs := []string{
@@ -1191,7 +1191,12 @@
 }
 
 func (c *configImpl) rbeAuth() (string, string) {
-	credFlags := []string{"use_application_default_credentials", "use_gce_credentials", "credential_file"}
+	credFlags := []string{
+		"use_application_default_credentials",
+		"use_gce_credentials",
+		"credential_file",
+		"use_google_prod_creds",
+	}
 	for _, cf := range credFlags {
 		for _, f := range []string{"RBE_" + cf, "FLAG_" + cf} {
 			if v, ok := c.environ.Get(f); ok {