pin the actions/checkout GitHub Action to a specific version

Author: fharper

.github/workflows/check-markdown.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout this repository
-        uses: actions/checkout@master
+        uses: actions/checkout@v4.1.4
 
       - name: Download .markdownlint.json
         uses: suisei-cn/[email protected]