flake.lib.nixosSystem: Allow nixpkgs.system to be set instead

flake.nix

@@ -26,6 +26,11 @@
                 ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
               system.nixos.revision = final.mkIf (self ? rev) self.rev;
             } ];
+          } // lib.optionalAttrs (! args?system) {
+            # 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;
           });
       });