Make a shared repo for sources

[alexlarsson]

Many upstreams have pretty shitty networks, or move and stuff. It would be nice if we had a shared cache of all the sources downloaded.

We would need a pre-build step that ran flatpak-builder --download-only, and exports the result over http to the builders. Then the builders need to add --extra-sources-url=SOURCE-URL to the flatpak-builder commandline to pick these up.