recording the builds that are part of a jobset evaluation. We need
this to be able to answer queries such as "return the latest NixOS
ISO for which the installation test succeeded". This wasn't previously
possible because the database didn't record which builds of (say)
the `isoMinimal' job and the `tests.installer.simple' job came from
the same evaluation of the nixos:trunk jobset.
Keeping a record of evaluations is also useful for logging purposes.
releases as a dynamic view on the database was misguided, since
doing thing like adding a new job to a release set will invalidate
all old releases. So we rename release sets to views, and we'll
reintroduce releases as separate, static entities in the database.
against the set of current builds for the job). This ensures that
the builds with the highest ID are what we want in the channel, even
in case of reverts.