installation: Fix a small leak of a FlatpakCollectionRef
installation: Fix a small leak of a FlatpakCollectionRef

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #3031
Approved by: mwleeds

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