Use ephemeral fuzzing-tc-config checkout instead of permanent repository

[jschwartzentruber]

We currently keep a git clone of fuzzing-tc-config repo and update it using a MozillaPulse trigger. We only care about the repo HEAD, never history, and the data size on disk is very small. There's no value in maintaining the repo state on disk between triggers.

We should remove the clone and use a temporary copy instead on every trigger (eg. using Github tar download). This eliminates one of our uses of persistent instance storage.