automerge-triggers.yml: fix overly broad permissions

Fixes https://github.com/Homebrew/homebrew-core/security/code-scanning/184

Author: carlocab

.github/workflows/automerge-triggers.yml

@@ -16,6 +16,8 @@ defaults:
 env:
   HOMEBREW_DISABLE_LOAD_FORMULA: 1
 
+permissions: {}
+
 jobs:
   check:
     if: >