GHA: disable permissions where missing

Author: vszakats

.github/workflows/ci.yml

@@ -16,6 +16,8 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
   cancel-in-progress: true
 
+permissions: {}
+
 jobs:
   DetermineMatrix:
     runs-on: ubuntu-latest