[arcanist] Untrack and ignore the arc liberate cache file

Summary:
From the documentation in https://secure.phabricator.com/book/phabcontrib/article/adding_new_classes/:

src/.phutil_module_cache This is a cache which makes "arc liberate" faster when you run it to update the library. You can safely remove it at any time. If you check your library into version control, you can add this file to ignore rules (like .gitignore).

Test Plan:
Arcanist still works:

arc lint

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D11674