Consider building an HTTP cache stored as a GitHub cache

[tidoust]

Via @dontcallmedom in #1356 (comment):

While we're looking at the cg-monitor, another idea from there maybe worth exploring is to build an HTTP cache and reuse it from one job to the next with GitHub caches.

[dontcallmedom]

since the changes seem to have been sufficient, and since the HTTP cache in CG monitor turns out flaky at best, let's close this issue; we can always re-consider if/when we get similar challenges in the future