feat(apk-polkit-rs): update AppStream data while updating repos

According to AppStream documentation, this is responsibility of the distributions. We currently ignore such advice and implement this both in GNOME Software and Plasma Discovery. This simple solution, however, allows to drop the use of the (buggy) external-appstream-urls, and if PD ever uses this project, also of the custom download they do.

In some way, this is part of the fix for https://gitlab.com/postmarketOS/pmaports/-/issues/1384, and makes us compliant with the AppStream specification. Additionally to the compliance, my other reason to do this is that the "external-appstream-urls" part of GNOME Software is terribly buggy and would be quite annoying to support situations like updates between major releases, which would require changing the gsettings override. This + cleanup is now possible with a 100LOC python script :D