Send notifications only on failure

[sjohnr]

Enhance the send-notification job to only trigger on a failure in a previous job.

Context:

The send-notification action sends notifications whenever it is invoked. There is a bug in GitHub Actions where the condition if: ${{ failure() }} does not work when a job in a reusable workflow fails. Further, if: ${{ !success() }} will always run if a previous job is skipped (skipped is apparently not success even though the docs say it is). This means we can't cause the notification only to be sent on failure.