Backport !1392 “Resolve "Crash under gs_flatpak_refine_app_unlocked()"” to gnome-42

Not entirely trivial backport of !1392 (merged) to gnome-42. There were a couple of conflicts where job functions exist on main but not on gnome-42. I think the resolutions were fairly straightforward though, and I’ve smoketested that gnome-software loads and can display a flatpak app page.