Notify slack when `scheduled:package-and-qa` fails

With `allow_failure: true` on a prerequisite job,
when the job fails it would be considered a success.
This results in the `schedule:package-and-qa` job being skipped.

To work around this, we always run
`schedule:package-and-qa:notify-failure`,
and look for a failed `schedule:package-and-qa` in the pipeline.