* tests/metrics.scm ("builds-per-day", "new-derivations-per-day"): Fix tests
results that were depending of the local time.
("evaluation-completion-speed"): Adapt to new unit.
* src/cuirass/metrics.scm (db-average-build-start-time-per-eval,
db-average-build-complete-time-per-eval, db-evaluation-completion-speed,
db-latest-evaluations): New procedures.
(%metrics): Add 'average-eval-build-start-time,
'average-eval-build-complete-time, 'evaluation-completion-speed.
(db-update-metrics): Update evaluation related metrics for the evaluations
added the past three days.
* tests/metrics.scm ("average-eval-build-start-time",
"average-eval-build-complete-time", "evaluation-completion-speed"): Add new
tests.
* tests/metrics.scm (nearest-exact-integer): Remove as unused.
("db-update-metrics"): Test that 'pending-builds metric is updated on
"db-update-metrics" call.