Using core.setOutput is much nicer than having to parse the json
"result" on the outside. This also avoids some very odd errors, when the
result can, for unknown reasons, *not* be parsed as JSON later on.
Also avoiding a bit of duplication between the "if mergeable" branches.
We don't need a separate workflow anymore, because we don't need to skip
dependent jobs on failures anymore. The biggest failure mode was
"conflict" previously, but we resolved that on the last commit. The
remaining failure modes are so rare, that it's OK to just fail the jobs
in this case instead of marking them as "skipped". Especially, because
the resolve-merge-commit job would have previously failed anyway.
By moving this to an action we avoid running separate jobs each time we
need the merge commit. This also makes the check list in PRs much
cleaner.
