ci: Fix flatpak job

The flatpak job is currently failing with "fuse: device not found, try 'modprobe fuse' first". This is because runners are now unprivileged, see https://mail.gnome.org/archives/desktop-devel-list/2020-February/msg00055.html. Let's add flatpak tag and run flatpak build with --user --disable-rofiles-fuse options to make it work again.