[Security] Pin GitHub Actions to a full-length commit SHA (#148)

Author: juliendoutre

.github/workflows/keep-alive.yml

@@ -17,7 +17,7 @@ jobs:
       contents: write # Required to push changes to the repository
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6.0.2
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Set up Git
         run: |