Improve GitHub Actions workflow security

Author: jwodder

.github/workflows/test.yml

@@ -12,6 +12,8 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.ref_name }}
   cancel-in-progress: true
 
+permissions: {}
+
 jobs:
   test:
     runs-on: ubuntu-latest
@@ -34,6 +36,8 @@ jobs:
     steps:
       - name: Check out repository
         uses: actions/checkout@v6
+        with:
+          persist-credentials: false
 
       - name: Set up Python
         uses: actions/setup-python@v6