Send GHA failure notifications to build cop channel automatically

[williamjallen]

Those who are subscribed to the build cop mailing list get emails about failing Jenkins jobs, but not for failing GitHub Actions jobs. These GitHub Actions failures can be reported to the build cop channel automatically via the official GitHub+Slack integration. If desired, something similar could be done for Jenkins as well.

[jwnimmer-tri]

I will defer the importance / priority of this to the actual buildcops (of which I'm not one). As a non-cop, I just check https://drake-jenkins.csail.mit.edu/view/Production/ every morning to survey our health. For buildcops who are supposed to triage more quickly than daily, I can see how a slack ping would help.