Oh, I guess that would just be `path.Join(worldDir(s.Id), "log.txt")`.

Also, actually, we probably want to us `os.Open()` so we can open the file in `O_APPEND` mode. Otherwise we'll truncate it if it exists already.

And maybe we want to add a couple blank lines & a "started server on ...." message to the start of the logs so we can tell the different generations when things crash.