bug#51787: GC takes more than 9 hours on berlin

[Mathieu Othacehe]

Hello,

> That’s indeed the case: the store is smaller than it used to be (but
> still 27 TiB), it’s GC’d more aggressively than before, and instead we
> rely on /var/cache/guix/publish for long-term storage.
>
> Perhaps we should go further and keep the store smaller though.

That's what I did with 93adf7aaa693d234ee13240e9f4ff22a2dfef599 on
maintenance. It increases the GC threshold to 15TiB. Let's see if it
brings some improvements.

Thanks,

Mathieu