Fix reverting synapse-admin to upstream instructions

Ref: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3521

CHANGELOG.md

@@ -12,6 +12,7 @@ If you'd like to switch back to the original synapse-admin software, you can do
 
 ```yml
 matrix_synapse_admin_docker_image: "{{ matrix_synapse_admin_docker_image_name_prefix }}awesometechnologies/synapse-admin:{{ matrix_synapse_admin_version }}"
+matrix_synapse_admin_docker_image_name_prefix: "{{ 'localhost/' if matrix_synapse_admin_container_image_self_build else matrix_container_global_registry_prefix }}"
 
 matrix_synapse_admin_version: 0.10.3