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/config.go

@@ -1205,6 +1205,21 @@ func (c *configImpl) rbeAuth() (string, string) {
 	return "RBE_use_application_default_credentials", "true"
 }
 
+func (c *configImpl) IsGooglerEnvironment() bool {
+	cf := "ANDROID_BUILD_ENVIRONMENT_CONFIG"
+	if v, ok := c.environ.Get(cf); ok {
+		return v == "googler"
+	}
+	return false
+}
+
+func (c *configImpl) GoogleProdCredsExist() bool {
+	if _, err := exec.Command("/usr/bin/prodcertstatus", "--simple_output", "--nocheck_loas").Output(); err != nil {
+		return false
+	}
+	return true
+}
+
 func (c *configImpl) UseRemoteBuild() bool {
 	return c.UseGoma() || c.UseRBE()
 }