ci: Notify when scheduled build fails
We setup a scheduled hourly build to run the operator against Contour. If that fails, we need a way to notify folks (email / slack, etc), that it's failing so we can take action.
Ex: https://github.com/projectcontour/contour-operator/actions/runs/614856080
xref github actions <> slack integration: https://github.com/marketplace/actions/slack-notify
Slack integration sounds cool, but we will need to go through the process to get that added to Kubernetes slack.
I think Email to the contour-maintainers CNCF list is probably deliverable faster, tbh.
xref: https://github.com/projectcontour/contour-operator/issues/243