flatpak: Make sure we are refreshing the appstream metadata

Merged Aleix Pol Gonzalez requested to merge work/apol/flatpak-ensure-refresh into Plasma/5.27

Do the 6 hours trick in case the remote wasn't found at all.

Note for future self when testing the feature, a good way to force the refresh is by running:

$ sudo touch -d "2 weeks ago" /var/lib/flatpak/appstream/flathub/x86_64/.timestamp

BUG: 448521 FIXED-IN: 5.27

Edited by Nate Graham

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