This allows for setting overlays at the home level while also inheriting
the global nixpkgs configuration. This should theoretically work exactly
like useGlobalPkgs while still allowing for flexibility at the home
level. See nix-community/home-manager#6172
for some reason, the toplevel derivation is different for some systems,
but nix-diff only shows one file being different, specifically the order
of some config relating to kernel modules. however, the produced system
is functionally identical. i have no idea what could be responsible for
this change.
