in windows-builder/builder/builder/remote.go [190:196]
func (bs *BuilderServer) GetServiceAccountEmail(projectID string) string {
if *bs.ServiceAccount == "default" || strings.Contains(*bs.ServiceAccount, "@") {
return *bs.ServiceAccount
}
//add service account email suffix
return fmt.Sprintf("%s@%s.iam.gserviceaccount.com", *bs.ServiceAccount, projectID)
}