temp/gnome-software: upgrade to 9999_git20200928 (possible to download stuff from it)

With this, we are on the latest from Purism and it is possible to download software with it without crashing (tested with Pinephone 1.2b).

I backported Alpine's Appstream override and I added pmOS Appstream which means we should see pmOS and Alpine's apps on the store. Since it's a gschema.override, I'm not sure if this belongs in the APKBUILD, but that's what Alpine did. What do you guys think?

NOTE FOR TESTERS
If you want to test the changes, it requires you to rebuild gnome-software's cache (the easiest way of doing that is rebooting).