Cache repository clone

[tchiotludo]

Now, we are doing a full clone of the repository, this can be a pain when the repository is a big one. We need to find a way to cache, avoid a full clone each time.
There is a concern about privacy & worker, so the subject is not easy, maybe rely on Cache from WorkerDirectory?

[Ben8t]

cc @fhussonnois ;) "ExecutionFiles" idea ringing again. Discussed with Anna briefly, it won't be prioritized for now