Do not persist credentials in checkout CI action

This mitigates a potential security risk. More info here:
<actions/checkout#485>

{{cookiecutter.repo_name}}/.github/workflows/main.yml

@@ -18,6 +18,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
 
       - name: Set up Python ${{ "{{" }} matrix.python-version {{ "}}" }} & PDM
         uses: pdm-project/setup-pdm@v4
@@ -39,6 +41,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
 
       - name: Validate links in Markdown files
         uses: JustinBeckwith/linkinator-action@v1