activitywatch: reduce test closure

This also reduces test flakiness.

tests/modules/services/activitywatch/basic-setup.nix

@@ -5,6 +5,7 @@ let stubPackage = config.lib.test.mkStubPackage { };
 in {
   services.activitywatch = {
     enable = true;
+    package = stubPackage;
     settings = {
       port = 3012;
       custom_static = { custom-watcher = stubPackage; };