Notify the infrastructure team when the untracked repos workflow fails

[marcoieni]

see #2580 (comment)

[cuba0001]

As I can see there are two possible ways to implement this:

1. Add a workflow-level notification job. This would notify the infrastructure team about any workflow failure. But the notification would probably only contain a link to the failed run.

2. Send the notification directly from check_untracked_repos in src/ci.rs. This would allow the notification to include the repository names, but it would not cover failures occurring before or after that point.

Or even both :)

Also, where exactly notifications should be sent to?

[Kobzol]

I would do it in the GHA workflow directly. For other tooling, we send messages to Zulip: https://github.com/rust-lang/josh-sync/blob/main/.github/workflows/rustc-pull.yml#L162.