enable nix flakes on darwin

nix/system/worktop/darwin-configuration.nix

@@ -164,6 +164,8 @@
     };
   };
 
+  nix.settings.experimental-features = [ "nix-command" "flakes" ];
+
   # Used for backwards compatibility, please read the changelog before changing.
   # $ darwin-rebuild changelog
   system.stateVersion = 4;