Support a custom FLUTTER_STORAGE_BASE_URL as the source for Flutter package downloads

[oravecz]

We cannot install Flutter from a Git cache, nor can we download Flutter from the https://storage.googleapis.com/flutter_infra_release/releases location. Instead, we maintain our own versions of Flutter SDKs on an internal location which we set our FLUTTER_STORAGE_BASE_URL to. We also maintain our own version of the Flutter metadata files with our own checksum values in the same manner as Google.

Looking at the source code, I thought that FVM would respect the FLUTTER_STORAGE_BASE_URL environment variable, but it does not.

Is it feasible to make a modification to the source code to ensure that FLUTTER_STORAGE_BASE_URL is used as the source for Flutter packages to be put in the local cache?

Originally posted by @oravecz in #571

[digoreis]

@oravecz this issue was resolved in this PR #593 , correct ?