temp/gnome-software: use external appstream data

Sync with Alpine's gnome-software APKBUILD. Note that the gnome-software-plugin-apk dependency is done as "install_if" in the gnome-software-plugin-apk APKBUILD now, to resolve a circular dependency.

Once we have postmarketOS specific appdata set up, we can add the link to org.gnome.software.gschema.override (future commit).

Related:

• https://gitlab.alpinelinux.org/alpine/aports/-/commit/3dbc1466fce6eeffd0ad69a96a20d8d5595b1d57
• #699 (closed)

Should be tested on a device / in qemu before merging.