fix(sliding-sync): use external syncv3 server url

roles/custom/matrix-sliding-sync/defaults/main.yml

@@ -76,7 +76,7 @@ matrix_sliding_sync_systemd_required_services_list: ["docker.service"]
 matrix_sliding_sync_systemd_wanted_services_list: []
 
 # Controls the SYNCV3_SERVER environment variable
-matrix_sliding_sync_environment_variable_syncv3_server: "{{ matrix_homeserver_container_url }}"
+matrix_sliding_sync_environment_variable_syncv3_server: "{{ matrix_homeserver_url }}"
 
 # Controls the SYNCV3_SECRET environment variable
 matrix_sliding_sync_environment_variable_syncv3_secret: ''