Backport fix for concurrent access to repository to v2021pre

Build fails when installing runtime while a runtime or an app is pushed to the repository. We can't use rsync to upload a runtime or an app.
This download commit metadata from the repository before build, then upload using ostree-push instead of rsync. The repository is then updated using flatpak build-update-repo.

Remove ci-flatpak-builder.yml which has been move to its own branch as it is not release specific.

Signed-off-by: Frédéric Danis frederic.danis@collabora.com