CI: Maintain a cache

[balacij]

The latest PR, #3674, took a lot of CI time, in part because it didn't hit a cache. While this ticket won't bring our build times down to 0, we should be able to shave off at least 10min, hopefully more, through caching.

@JacquesCarette previously sent me a link to a GitHub Actions template for Haskell. It contained code for how to do this.

The caches are evicted after 7 days without use and aren't overwritten with each usage. I think we can add an automated action run every 8th day to make sure that, when development stalls and eventually returns, we always have a cache to use.

[balacij]

Cache hits have been consistent now. No need for this ticket anymore.