feat(nix): don't install Xcode via mas. It takes so long

Signed-off-by: Seongmin Lee <[e-mail]>

nix/darwin.nix

···
       Pages = 409201541;
       Tampermonkey = 6738342400;
       Things3 = 904280696;
-      Xcode = 497799835;
+      # Xcode = 497799835;
     };
   };
 }