Allow merge groups to trigger the self-zizmor workflow

Author: guicaulada

.github/workflows/self-zizmor.yaml

@@ -6,6 +6,8 @@ on:
   pull_request:
     paths:
       - ".github/**"
+  merge_group:
+    types: [checks_requested]
 jobs:
   zizmor-check:
     name: Check whether there are things to scan