Lake build with dependencies require internet access

[lecopivo]

Having mathlib4 as a dependency seems to break lake build if you are offline.

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Reproduce:

In a clean directory:

lake init Test

Add the following to lakefile.lean

require mathlib from git
  "https://github.com/leanprover-community/mathlib4.git"@"8f609e0ed826dde127c8bc322cb6f91c5369d37a"

Run lake build for initial build and initialization of mathlib4.

Disconnect from the internet and run lake build again, results in:

error: stderr:
fatal: unable to access 'https://github.com/leanprover-community/mathlib4.git/': Could not resolve host: github.com
error: git exited with code 128

---

Tested on: leanprover/lean4:nightly-2022-07-11

[lecopivo]

Thanks for fixing this!

[pechersky]

I've hit this issue again, with mathlib4 and nightly 2022-10-20, when offline. This prevents the lakefile from parsing, which prevents me from being able to work on porting even when I'm offline.