[RFC] CI: keep contrib tarballs between jobs

Rather than downloading the same tarballs in 20 jobs on a few machines, we could use Gitlab artifacts (or other systems) to keep the content of the contrib/tarballs between runs. This would be nicer to the remote sources and avoid random connection issues with such servers.

The size of my folder is 2.5 GB. Maybe that's big for artifacts.

Bonus: it could be a (secondary) source for tarballs Videolan keeps around.

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