workflows/eval: fail hard without target run

[wolfgangwalther]

Without a target run, we won't get any rebuild labels, rebuild counts or
maintainer pings. This might have been correct before #373935, but by
now we run eval on all commits on the target branch, so we should treat
it as a failure if we can't find the run.

As mentioned in #355847 (comment).

Resolves #378595

Things done

• Fits CONTRIBUTING.md.

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Add a 👍 reaction to pull requests you find important.

[infinisil]

Let's have this, especially also because there's a bunch of these failures recently, which are very opaque to the PRs not getting processed due to it

[nixpkgs-ci]

Successfully created backport PR for release-24.11:

• [Backport release-24.11] workflows/eval: fail hard without target run #379302

[wolfgangwalther]

especially also because there's a bunch of these failures recently, which are very opaque to the PRs not getting processed due to it

They seem to indicate a problem with GHA being overloaded, imho. They roughly appeared at the same time * when we enabled running eval on all commits on master / staging. This is probably pure coincidence and not related, but I still wonder whether anyone knows more about the source of those failures and whether we can do anything about them?

There were times when those happened on almost every push...

* to be fair, that was also the time when GitHub Actions announced the new public beta of their arm linux runners. So maybe stuff for GHA actions changed on a bigger scale and this is entirely unrelated to us.