FlatpakBackend: Update appstream cache before initializing
We were launching the job to update the appstream cache after initializing the backend. As a result, the updated versions were not used until the next launch of discover.
This should finally fix the version numbers shown for flatpaks on the Updates page.
I've been testing this for a couple of days with no regressions but I'm not so used to the codebase as to know if it's the best fix.
Edited by Ismael Asensio