Use prebuilt container images for matrix-sliding-sync on ARM64

As mentioned in https://github.com/matrix-org/sliding-sync/issues/31#issuecomment-1640321110
images are available for arm64 already.

group_vars/matrix_servers

@@ -3927,7 +3927,7 @@ matrix_sliding_sync_hostname: "{{ matrix_server_fqn_matrix }}"
 
 matrix_sliding_sync_path_prefix: /sliding-sync
 
-matrix_sliding_sync_container_image_self_build: "{{ matrix_architecture not in ['amd64'] }}"
+matrix_sliding_sync_container_image_self_build: "{{ matrix_architecture not in ['amd64', 'arm64'] }}"
 
 matrix_sliding_sync_container_additional_networks: |
   {{