Lists items are not directly accessible like attributes in attrsets are.
This makes it hard to represent their address in `UPDATE_NIX_ATTR_PATH`
environment variable passed to update scripts.
Given that I only introduced list support for `gnome3` attribute set
and we stopped using them there, let’s remove the list support again.
NixOS modules are better place for package collections anyway.
This was meant to go in with https://github.com/NixOS/nixpkgs/pull/98304
but got accidentally omitted somehow.
