LocalDatastoreHelper fails to start if used in an environment that cannot access the "gcd" GCS bucket #376

Is your feature request related to a problem? Please describe.
When attempting to use the Downloadable emulator, LocalDatastoreHelper downloads from a hardcoded URL to the file in the "gcd" GCS bucket:

When used in an environment that cannot access the "gcd" GCS bucket, LocalDatastoreHelper fails to start (e.g. when running in a GCE VM protected by a VPC-SC perimeter).

Describe the solution you'd like
If I could provide the emulator file URL in an environment variable, I could copy the file from the "gcd" bucket into another bucket that is accessible from the restricted environment and use that URL.

Describe alternatives you've considered
N/A