DT-757: fix update notifications
requested to merge sergio-costas/gnome-software:DT-757-fix-notifications-in-snap-store into snap-store-41
Snap-Store uses the GLib notification API, which has some requirements that, currently, aren't possible in a snap.
James proposed as a quick fix to set GNOTIFICATION_BACKEND as "freedesktop", thus using the base interface.
This MR fixes https://bugs.launchpad.net/snap-store-desktop/+bug/1990947