Provide path to .env file at build time

[SharbelOkzan]

Motivation:
Facilitate working with flavors.

Goal:
Have the possibility to pass the path to the .env file with flutter build and flutter run. It will still default to /.env if not provided.
Users may utilize this feature by having multiple env files and executing:

flutter run --flutter-config-path=/.env.stage
# or
flutter run --flutter-config-path=/.env.prod

Does that exist? If not, is it theoretically possible?

[dougalg]

Yes, this seems like really useful functionality to me. If this is interesting to the maintainers, maybe I could try making a PR