Fix broken 'prune old images' step

Author: Bastian

.github/workflows/deploy-to-google-cloud.yml

@@ -80,7 +80,7 @@ jobs:
         gcloud compute instances add-metadata $GCE_INSTANCE \
           --zone "$GCE_INSTANCE_ZONE" \
           --metadata=startup-script="#! /bin/bash
-            docker image prune -af
+            docker image prune -af"
     - name: Purge GCR images
       run: |-
         gcloud container images list-tags gcr.io/$PROJECT_ID/$GCE_INSTANCE-image \