✏️ host: tau: Fix misleading task name

Task here is to check the auth dir, not the data dir, that's a different
one.  Rename the used variable while at it.

Fixes: f539a42024 ("Add a docker registry")

host-tau.yml

@@ -86,11 +86,11 @@
     - name: Check if Docker Registry auth dir exists
       ansible.builtin.stat:
         path: "{{ data_dir }}/registry/auth"
-      register: docker_dir
-    - name: Fail if docker registry data dir does not exist
+      register: docker_registry_auth_dir
+    - name: Fail if Docker Registry auth dir does not exist
       ansible.builtin.fail:
         msg: "Docker Registry auth dir is missing, please restore from the backup!"
-      when: not docker_dir.stat.exists
+      when: not docker_registry_auth_dir.stat.exists
     - name: Ensure the Docker Registry data directory exists
       # This may not be part of the backup
       ansible.builtin.file: