We were overriding the `matrix_bot_matrix_registration_bot_matrix_homeserver_url` variable via `group_vars/matrix_servers`
to point it to the container-local homeserver URL, but this variable was not being used at all.

The actual variable name (which was being used) is `matrix_bot_matrix_registration_bot_api_base_url`.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3898