[cd] Send Slack notification if publish fails (#96304)
Failing to publish (especially a partial publish) leaves Next.js in a
corrupted state that needs to be addressed immediately. However, we only
created alerts when deploy tests failed which requires a successful
publish.
Now we send special message to the same channel as broken canary and
deploy tests go.
This won't catch a failing deploy-target job. We'll see if deploy-target
also frequently fails/flakes.
