We need to optimise the buildbot attrs for nix-eval-jobs, however this
doesn't make sense for `nix flake check` or `nix flake show`.
Now that we aren't using the `checks` output, we don't _need_ to
restrict ourselves to a flat set of test derivations anymore.
This also simplifies our `tests` command, and means we no longer need to
pre-compute the test attr names.
db7c5364a5