bug#33310: Cuirass: Evaluations adding no new builds are displayed as "failing"

["Clément Lassieur" <clement@lassieur.org>]

Hi,

Evaluations adding no new builds are displayed as "failing".  I know
this was done on purpose, but I think the check should be done on the
number of jobs returned by the evaluator, rather than on the number of
builds registered in the database.

There is a difference because derivations that are already in the
database won't be added again.

In my case (https://cuirass.lassieur.org/jobset/guix-manifest-clem), 70%
of the evaluations add no new builds, and none of them are failing.

--8<---------------cut here---------------start------------->8---
    (with-store store
      (let ((jobs (evaluate store spec eval-id checkouts)))
        (log-message "building ~a jobs for '~a'"
                     (length jobs) name)
        (build-packages store jobs eval-id)))
--8<---------------cut here---------------end--------------->8---

Thus if (zero? (length jobs)) here, we should mark the corresponding
evaluation as failing.

Thanks,
Clément