nixos/matrix-synapse: fix .well-known delegation of federated traffic

Synapse is reverse-proxied on ${fqdn} not ${config.networking.domain} and
the .well-known delegation info must point to the domain on which synapse
is hosted, see https://matrix-org.github.io/synapse/latest/delegate.html

nixos/modules/services/matrix/synapse.md

@@ -31,7 +31,7 @@ let
     "m.homeserver".base_url = "https://${fqdn}";
     "m.identity_server" = {};
   };
-  serverConfig."m.server" = "${config.services.matrix-synapse.settings.server_name}:443";
+  serverConfig."m.server" = "${fqdn}:443";
   mkWellKnown = data: ''
     add_header Content-Type application/json;
     add_header Access-Control-Allow-Origin *;