Backport !712 “Resolve "flatpak: Download appstream data on demand when missing"” to gnome-40
requested to merge pwithnall/gnome-software:backport-712-flatpak-appstream-refresh-gnome-40 into gnome-40
The flatpak can have missing appstream data for example when a remote is added and Software has disabled automatic update. In such case, download the appstream data on demand.
Closes #1217 (closed)
Trivial backport of !712 (merged) to gnome-40.