buildkite: realize extraSteps.command only at step execution time

Opened by sterni at 2022-02-17T11·21+00

While we have the needsOutput mechanism, other dependencies of the extra step script will be built or substituted at eval time which is not great. As extraSteps become more prevalent, we'll end up fetching more and more dependencies on channel bumps.

We could also take this opportunity to fix bufcheck which also exhibits this problem.