Fix /api/latestbuilds ordering.

Evaluations are added sequentially to database but builds are not always
registered nor performed in order. This means that a build corresponding to an
older evaluation can be returned first if it was completed last. Order by
descending evaluation id to prevent it.

* src/cuirass/database.scm (db-get-builds): Add "evaluation" order.
* src/cuirass/http.scm (url-handler): Order latestbuilds by descending
evaluation number.