tb-build-06 is nearly out of free disk

We just got an alert for this host. The disk nearly filled up but then some space got freed up so the alert cleared out.

On /srv currently looks like this:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb        787G  753G   28G  97% /srv

from what I can see most of the disk space is being used by torbrowser nightly builds, so if we could clear out some older builds that could help freeing up some disk, but:

there's a 100Gb swapfile on that disk. there's a peak of usage up to 40Gb of swap every day so apparently it's necessary to have that huge file, but maybe we could cut it down by half, 40% less?