Keep failed logs for a longer period, ideally for a configurable time

[praiskup]

For folks doing mass rebuilds often spend a nontrivial amount of time submitting the builds, iterating, etc. But then analyzing the failed build logs is the most expensive part, but we give them only 14 days by default till the logs are removed.

[praiskup]

If not configurable, I believe we could at least have a better default - like 3 months for failed logs (as long as the build driectory with logs is <= 100MB, i.e. builder-live.log.gz + backend.log.gz + ...)?