tracktrain: don't start before network.target

i hope this is the correct option; the last one apparently wasn't
enough.

services/tracktrain.nix

@@ -74,6 +74,7 @@ in
 
         description = "tracks trains, hopefully";
         wantedBy = [ "multi-user.target" ];
+        requires = [ "network.target" ];
         after = [ "network.target" ];
         serviceConfig = {
           Type = "simple";