flake: allow specifying the lib argument to nixosSystem

Some configs rely on being able to pass their own lib argument into
nixosSystem, for instance in order to add their own additional overlay
to nixpkgs.lib.
This was broken by 039f73f134.

flake.nix

@@ -21,12 +21,13 @@
 
         nixosSystem = args:
           import ./nixos/lib/eval-config.nix (
-            args // { inherit (self) lib; } // lib.optionalAttrs (! args?system) {
+            {
+              lib = final;
               # Allow system to be set modularly in nixpkgs.system.
               # We set it to null, to remove the "legacy" entrypoint's
               # non-hermetic default.
               system = null;
-            }
+            } // args
           );
       });