Slack notifications for unsuccessful cloud builds

[stephen-soltesz]

Cloud builds regular fail for transient reasons that we do not detect until much later because there is no notification mechanism for failed builds. This guide describes how to setup Slack notifications for Cloud builds.

• https://cloud.google.com/build/docs/configuring-notifications/configure-slack

Their examples filter on SUCCESS. But we could setup notifications for any non-SUCCESSful build, and send to the appropriate slack alert channel, #alerts-staging or #alerts-oti.

Available Build.Status enum values: https://cloud.google.com/build/docs/api/reference/rest/v1/projects.builds#Build.Status