Stop trying to hit a specific directory with mktemp

Just use the configured temporary directory in the environment.  This is less
great but it's still okay because:

* It's our system
* The deploy key has limited capabilities

.gitlab-ci.yml

@@ -84,8 +84,7 @@ system-tests:
     - |
       # The environment variable is configured with GitLab using Terraform so
       # we can retain some bare minimum level of confidentiality.
-      KEY_DIR="$(mktemp -d -p "${XDG_RUNTIME_DIR}-deploy_key")"
-      KEY_PATH="${KEY_DIR}/deploy_key"
+      KEY_PATH="$(mktemp -d)/deploy_key"
       base64 --decode "${PRIVATESTORAGEIO_STAGING_SSH_DEPLOY_KEY}" > "${KEY_PATH}"
 
     # Update the deployment