GitHub Workflows security hardening (graphql#3751)

Author: sashashura

.github/workflows/mutation-testing.yml

@@ -3,6 +3,8 @@ on:
   workflow_dispatch:
   schedule:
     - cron: '0 0 * * *' # run once every day at 00:00 UTC
+permissions:
+  contents: read # to fetch code (actions/checkout)
 jobs:
   lint:
     name: Run mutation testing