Allows splitting the workflow file in the next step.

(cherry picked from commit 368a2d6)

This runs the "check owners" job as part of the main PR workflow, with
multiple advantages:
- The job doesn't run anymore when undrafting a PR, where it's not
required, because the OWNERS file's contents didn't change.
- A valid OWNERS file is now a requirement to merge a PR.
- The OWNERS file is always checked on the exact same test merge commit
that the remainder of the workflows are running on as well.

(cherry picked from commit 15be04f)

Changes to this file already trigger a run of this workflow via the PR
workflow, we don't need to run this *another* time.

(cherry picked from commit 7f4dd7e)

These two jobs mostly used the same code anyway, so they can easily run
in the same job. This uses fewer resources and avoids running (and
failing) the codeowners review job on Pull Requests from development
branches like staging-next or haskell-updates.

It also opens the door for more improvements later on, for example it
will be much easier to implement a shared reviewer limit for both owners
and maintainers now.

(cherry picked from commit ddf81de)