ci: Use GNOME's flatpak CI template
GNOME has started issuing templates for Flatpaks in the CI. Use these to simplify our own CI script and to benefit if any features pops up in there.
Edited by Niels De Graef
GNOME has started issuing templates for Flatpaks in the CI. Use these to simplify our own CI script and to benefit if any features pops up in there.