Enable self-building of matrix-synapse-admin on non-amd64

Related to #658 (Github Issue).

group_vars/matrix_servers

@@ -1024,6 +1024,8 @@ matrix_synapse_admin_enabled: false
 # Synapse Admin's HTTP port to the local host.
 matrix_synapse_admin_container_http_host_bind_port: "{{ '' if matrix_nginx_proxy_enabled else '127.0.0.1:8766' }}"
 
+matrix_synapse_admin_container_self_build: "{{ matrix_architecture != 'amd64' }}"
+
 ######################################################################
 #
 # /matrix-synapse-admin