ci: fix workflow permissions (#7366)

cc https://github.com/paritytech/ci_cd/issues/1101

.github/workflows/check-runtime-migration.yml

@@ -16,6 +16,8 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
 
+permissions: {}
+
 jobs:
   preflight:
     uses: ./.github/workflows/reusable-preflight.yml