CI: try to make the flatpak tests run again
Half a year ago they broke due to some changes of either the gnome-runtime-images docker images, or the contained flatpak, or something else: gnome-runtime-images#7
This creates a fake dbus system bus so flatpak run doesn't fail.
After this it turned out that pip install is now completely broken in the sdk and it installs packages to a place where it can't find them afterwards, so just use a venv instead.
Finally the pygobject test suite still crashes at some point, but at least it's better then before.