ci: pin actions to SHA refs

Author: estebany-qd

.github/workflows/deploy.yaml

@@ -18,7 +18,7 @@ jobs:
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
     # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-    - uses: actions/checkout@v6
+    - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
     - name: Build backend
       run: |
         cd $GITHUB_WORKSPACE