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.
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