OCI: Fix signature checks on updates
OCI: Fix signature checks on updates

We can't support updating to a specified digest, we must
always look in the index so that we can find the signature.

handsome-feng/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak