Read the deploy key from a file instead of the environment

.gitlab-ci.yml

@@ -87,7 +87,7 @@ deploy-to-staging:
     - |
       # The environment variable is configured with GitLab using Terraform so
       # we can retain some bare minimum level of confidentiality.
-      echo "${PRIVATESTORAGEIO_STAGING_SSH_DEPLOY_KEY}" > "${PWD}"/deploy_key
+      base64 --decode "${PRIVATESTORAGEIO_STAGING_SSH_DEPLOY_KEY}" > "${PWD}"/deploy_key
 
     # Update the deployment
     - |