Convert tatoeba port number to string in flake

flake.nix

@@ -86,7 +86,7 @@
             wantedBy = [ "network.target" ];
             serviceConfig = {
               ExecStart = ''
-                ${cfg.package}/bin/tatoeba --port ${cfg.port}
+                ${cfg.package}/bin/tatoeba --port ${builtins.toString cfg.port}
               '';
               Restart = "always";
               DynamicUser = true;