Simplify our CI setup by moving cache population and using Magic Cache

- Our CI setup was made overly complex because of the steps that
  populate the cache. Moving them out of the main CI in separate jobs
  should help.
- Also we now keep a cache per week as pushing daily caches to S3 will
  eat a lot of space.
- Finally, we use the Magic Cache feature of RunsOn which should
  transparently use the cache implementation that makes the most sense.

Author: gsmet