repo-updater: Only purge repos for 1 hour on saturday night
Created by: keegancsmith
We can remove the shouldKeep behaviour. We intentionally keep the 10min sleep behaviour so we can run multiples times on Saturday night (in case of an ephemeral error).
See the linked issue for motivation for this approach to purge repositories.
Part of https://github.com/sourcegraph/sourcegraph/issues/2572