I'm deploying a Cloud Function which interacts with Google Drive. As far as I can tell, there's no way to use the implicit Cloud Function credentials or related environment variables to authorize with Drive, so I've had to resort to either including the credentials.json service account key file in the Cloud Function bundle or by setting an environment variable using that same key file during deployment of the Cloud Function.
It'd seem to me that you would be able to use the Cloud Function's implicit service account and credentials in order to achieve this.