As we expand use of the new fetch-deps command in CI, we should add a flag called something like --minimize-network that will do steps explicitly offline when possible. Most notably, we should plumb it through to pass --offline any pub get commands that we run, so that we can pub get in online mode only in fetch-deps, and then in offline mode for the rest of the run.

(I suggest minimize-network rather than offline to make it less misleading; we won't actually be able to do everything offline. E.g., I don't see a fully offline mode for the pod commands run by flutter bulids.)