darwin.builder: Fix gratuitous rebuilds

See the discussion starting here:

#206951 (comment)

The `darwin.builder` derivation had a gratuitous dependency
on the current Nixpkgs revision due to
`config.system.nixos.revision`.  Setting the revision explicitly
to null fixes this problem and prevents the derivation from being
rebuilt on every change to Nixpkgs.

(cherry picked from commit 474198f)

nixos/modules/profiles/macos-builder.nix

@@ -93,7 +93,12 @@ in
         };
       });
 
-  system.stateVersion = "22.05";
+  system = {
+    # To prevent gratuitous rebuilds on each change to Nixpkgs
+    nixos.revision = null;
+
+    stateVersion = "22.05";
+  };
 
   users.users."${user}"= {
     isNormalUser = true;