flake: inject extended 'lib' into NixOS config

Somehow it works just fine in my `home-manager` configuration, I assume it is
using the system `nixpkgs` and its `lib` attribute that I extended. Whereas the
NixOS system must be injected with the extended one intentionally.

flake.nix

@@ -39,6 +39,10 @@
         modules = defaultModules ++ [
           (./. + "/${name}.nix")
         ];
+        specialArgs = {
+          # Use my extended lib in NixOS configuration
+          inherit lib;
+        };
       };
     in
     eachDefaultSystem