workflows: fix and cleanup permissions (graphql#3797)

Author: IvanGoncharov

.github/workflows/mutation-testing.yml

@@ -3,12 +3,13 @@ on:
   workflow_dispatch:
   schedule:
     - cron: '0 0 * * *' # run once every day at 00:00 UTC
-permissions:
-  contents: read # to fetch code (actions/checkout)
+permissions: {}
 jobs:
   lint:
     name: Run mutation testing
     runs-on: ubuntu-latest
+    permissions:
+      contents: read # for actions/checkout
     steps:
       - name: Checkout repo
         uses: actions/checkout@v3