background-worker: Speed up index cloning by 3x

[Turbo87]

Previously we were using libgit2 to clone the index, which regularly took 2-3 minutes. This PR switches us over to using the git CLI, which appears to be significantly faster and clones the index in just over 30 seconds.

[Eh2406]

cc @Byron, sounds like a good place for gitoxide some day.

[Byron]

Thanks for the heads-up! There is nothing stopping you from using gitoxide for this today, and depending on the hardware the worker runs on it should be a little faster, but with this repo no more than a second even with ample cores. The repository doesn't actually spend a lot of time in the delta-resolution phase which is the only phase that can be sped up meaningfully.

Since it's a background worker, one would also not be interested in progress here so using the git CLI seems like a good choice.

The only advantage one would have with gitoxide is to more easily detect network-related errors to retry, but then again, a simple retry-three-times logic might also be sufficient here.

[Turbo87]

@jtgeibel FYI I've tested this out on staging and it appears to work fine :)