Cuirass exposes evaluation logs

[Ludovic Courtès]

Hello Guix!

Good news everyone!  Cuirass now keeps logs of evaluations and exposes
them over HTTP!  So when an evaluation fails, you can now click on the
red cross to peek at the log.  More generally, you can view an
evaluation log by going to:

  https://ci.guix.gnu.org/eval/ID/log/raw

where ID is the numerical ID of the evaluation.

Logs are available only for evaluations performed after like, now.
They’re stored gzipped in /var/log/cuirass.

(Yeah it’s a bit ridiculous that we’ve had to make do without this
feature for so long, but at least now you no longer need to ask someone
with SSH access to berlin to parse /var/log/cuirass.log in search of
hints.  :-))

Feedback welcome!

Ludo’.