ci: Add sanity check for artifacts to post-build job

ci: Add sanity check for artifacts to post-build job

Check if expected artifacts have been created for every workflow run,
not only for tagged commits. While this is just a rudimentary
sanity check, it would at least cause a CI failure if one of the
build jobs didn't produce artifacts for some reason.

A nice side effect is that unlike the former create-release job,
the new post-build job is not completely skipped for non-tagged
commits, and thus shows up as successful for every good CI run
(even though the individual steps required for creating a draft
release are skipped, of course).
This causes the GitHub UI to automatically collapse the (giant)
list of checks after they are all complete.