Firebase: switch from `FIREBASE_TOKEN` (deprecated) to `GOOGLE_APPLICATION_CREDENTIALS`

[Jolg42]

⚠  Authenticating with `FIREBASE_TOKEN` is deprecated and will be removed in a future major version of `firebase-tools`. Instead, use a service account key with `GOOGLE_APPLICATION_CREDENTIALS`: https://cloud.google.com/docs/authentication/getting-started

Found in logs of platforms-serverless (firebase-functions, library, ubuntu-20.04)