Add sentinel job

[angerman]

We often need to know if all jobs have been built. Currently we don't at best we could check for hydra-statuses and ensure that no pending ones are left. That's fairly unergonomic.

https://determinate.systems/posts/hydra-deployment-source-of-truth might help with the aggregate job. The question is whether or not that aggregate job ends up pulling all the constituents onto the aggregate building machine.