[CI] Align Flatpak job with KDE
- Move and rename manifest according to how KDE does it
- Use the current directory instead of a remote repository (its kind of already done like this but in a much more hacky way)
- Remove now unused local_flatpak.py script
- Build Potrace in the manifest instead of using the build in version
- Bundle flatpak repo as artifact
- Split appstream validation to a seperate job
- Disable validation job for now due to Flatpak bug
See also https://invent.kde.org/packaging/flatpak-kde-applications/-/blob/master/README.md