nixos: Add 20.09 release
It introduces a new GNOME variant, which is preferred over Plasma 5 variant.
The update script worked almost flawlessly, only had to add the new gnome variant to 20.03 temporarily for it to create nixos-gnome-20.09 isodata, since the next stable XML file is generated from the previous one, even though in reality stable is always branched off from the rolling unstable branch.