If the `ci` options are a set of options rather than a single option,
then the module system cannot lazily override it via `mkForce`.
In practice, this means that the dev partition gets evaluated strictly
when the module system "pushes down" the `ci` definitions to its
options.
Instead, we must ensure that each attribute listed in `partitionedAttrs`
corresponds to an actual option which can be directly defined, with no
"pushing down" required.
Fixes#3532
Adds top-level and per-system `ci.buildbot` option for defining tests to
build on buildbot.
These are exposed as the `ci.buildbot.«system»` flake output.
