Fix concurrent access to repository

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 commits from the repository before build, it should be fully downloaded so we can't use --commit-metadata-only of ostree pull.
Then build result is uploaded using ostree-push instead of rsync.
The repository is then updated using flatpak build-update-repo.

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