Draft: new caching system in CI
Instead of the gitlab-prebuilds tarball, this is closer to upstream pipelines: a first job provides the dependencies either as artifacts or by updating a big cache.
The main advantage is that we had no reliable way to update the tarball only when needed (
changes: is an approximation that's actually wrong in many important cases).
Thanks to this, we should not need to worry about changing cache keys, or updating the tarball separately any more. The price to pay is some more latency (most of which is actually time to first clone, which is another kind of issue anyway).