Security hardening: lock community action versions

As there's no lockfile for GitHub Actions, specify the commit SHAs to
use for community actions so that changes can be reviewed before using a
new version of a particular action.

.github/workflows/deploy.yml

@@ -20,17 +20,17 @@ jobs:
 
     steps:
       - name: Checkout the code
-        uses: actions/checkout@v2
+        uses: actions/checkout@a81bbbf
 
       - name: Add the deployment SSH key
-        uses: shimataro/ssh-key-action@v2
+        uses: shimataro/ssh-key-action@6f350ca
         with:
           key: ${{ secrets.SSH_PRIVATE_KEY }}
           name: id_rsa
           known_hosts: ${{ secrets.SSH_KNOWN_HOSTS }}
 
       - name: Cache dependencies
-        uses: actions/cache@v1
+        uses: actions/cache@d974700
         with:
           path: tools/ansible/.roles
           key: dependencies-composer-${{ hashFiles('tools/ansible/requirements.yml') }}