oci: Query for org.flatpak.ref label instead of annotation
oci: Query for org.flatpak.ref label instead of annotation

This is not compatible with the old stuff, but works with registries
that only support the docker manifest format.

Closes: #2978
Approved by: alexlarsson

flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak