Create artifact cache for buildstream tutorial builds

Background

Since !1300 (closed) will add a number of projects with insignificant build time to the basic buildstream tutorial, and we don't want a new user to wait 2 hours to get through building, we need an artifact cache for the tutorial projects.

The artifacts for these projects total about 5GB in size at most, but will need to be replicated for each cache key change (I know we say don't change those, but it's happened a few times now). We might also need more as tutorial pages are added.

This also broadens the scope of the tutorial a little bit, moving artifact caches into focus.

Task description

• Set up an artifact cache server
• Tie updating this cache into CI (preferably as part of !1300 (closed))

Acceptance Criteria

• We have a buildstream artifact cache that is consumed by the buildstream tutorial

---