This hasn't been used in a long time (Guix uses its own CI system),
and it probably doesn't work anymore.

(cherry picked from commit 23c9ca3)

(cherry picked from commit cf961ac)

(cherry picked from commit 950e8be)

(cherry picked from commit e7f2139)

(cherry picked from commit 639c660)

(cherry picked from commit be8eb9d)

In the past, jobsets which are automatically evaluated are evaluated
regularly, on a schedule. This schedule means a new evaluation is
created every checkInterval seconds (assuming something changed.)

This model works well for architectures where our build farm can
easily keep up with demand.

This commit adds a new type of evaluation, called ONE_AT_A_TIME, which
only schedules a new evaluation if the previous evaluation of the
jobset has no unfinished builds.

This model of evaluation lets us have 'low-tier' architectures.

For example, we could now have a jobset for ARMv7l builds, where
the buildfarm only has a single, underpowered ARMv7l builder.
Configuring that jobset as ONE_AT_A_TIME will create an evaluation
and then won't schedule another evaluation until every job of
the existing evaluation is complete.

This way, the cache will have a complete collection of pre-built
software for some commits, but the underpowered architecture will
never become backlogged in ancient revisions.

Declarative jobsets were broken by the Nix update, causing
nix cat-file to break silently.

This commit restores declarative jobsets, based on top of a commit
making it easier to see what broke.

Fix declarative jobsets due to `nix` command being hidden

 hydra-evaluator: add a 'ONE_AT_A_TIME' evaluator style

Fixes #718.