Use a `latest` symlink to point to the last written log file

We write log files like tor-weather-23-09-25-11:45:51.log that contain issues encountered during our Onionoo job run. We want to monitor the related onionoo.service and onionoo.timer jobs as they sometimes break silently. Ideally, we'd re-use recent work recently done in tpo/network-health/metrics/monitoring-and-alerting#32 (closed) for the TorDNSEL service. Therefore, we should create a symlink, latest, always pointing to the last written log file.

Once that's in place we can make progress on tpo/network-health/metrics/monitoring-and-alerting#24 (comment 2913883).