CI: reduce bandwidth for git pull

Over the last 7 days, git pulls represented a total of 1.7 TB.

On those 1.7 TB, we can see:
- ~300 GB for the CI farm on hetzner
- ~730 GB for the CI farm on packet.net
- ~680 GB for the rest of the world

We can not really change the rest of the world*, but we can
certainly reduce the egress costs towards our CI farms.

Right now, the gitlab runners are not doing a good job at
caching the git trees for the various jobs we make, and
we end up with a lot of cache-misses. A typical pipeline
ends up with a good 2.8GB of git pull data. (a compressed
archive of the mesa folder accounts for 280MB)

In this patch, we implemented what was suggested in
https://gitlab.com/gitlab-org/gitlab/-/issues/215591#note_334642576

- we host a brand new MinIO server on packet
- jobs can upload files on 2 locations:
  * git-cache/<namespace>/<project>/<branch-name>.tar.gz
  * artifacts/<namespace>/<project>/<pipeline-id>/
- the authorization is handled by gitlab with short tokens
  valid only for the time of the job is running
- whenever a job runs, the runner are configured to execute
  (eval) $CI_PRE_CLONE_SCRIPT
- this variable is set globally to download the current cache
  from the MinIO packet server, unpack it and replace the
  possibly out of date cache found on the runner
- then git fetch is run by the runner, and only the delta
  between the upstream tree and the local tree gets pulled.

We can rebuild the git cache in a schedule job (once a day
seems sufficient), and then we can stop the cache miss
entirely.

First results showed that instead of pulling 280MB of data
in my fork, I got a pull of only 250KB. That should help us.

* arguably, there are other farms in the rest of the world, so
hopefully we can change those too.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5428>