darwin: add Nix package to activation $PATH

The fix for https://github.com/nix-community/home-manager/issues/2178
did not apply the patch to nix-darwin too.

See: https://github.com/nix-community/home-manager/issues/2178#issuecomment-1029015498
(cherry picked from commit 1fa809f783)

nix-darwin/default.nix

@@ -29,6 +29,10 @@ let
 
           home.username = config.users.users.${name}.name;
           home.homeDirectory = config.users.users.${name}.home;
+
+          # Make activation script use same version of Nix as system as a whole.
+          # This avoids problems with Nix not being in PATH.
+          home.extraActivationPath = [ config.nix.package ];
         };
       })
     ] ++ cfg.sharedModules;