fix(synapse): use correct key in hostvars

roles/synapse/tasks/check.yml

@@ -17,8 +17,8 @@
     msg: "Required variable '{{ item }}' is undefined!"
   loop: "{{ synapse_required_variables }}"
   when: >-2
-    item not in hostvars[ansible_host]
-    or hostvars[ansible_host][item] | length == 0
+    item not in hostvars[inventory_hostname]
+    or hostvars[inventory_hostname][item] | length == 0
 
 - name: Ensure conditionally required variables are given
   ansible.builtin.fail: