Notify when the cache is broken

[ThetaSinner]

When the cache workflows fail, end users are affected. We need to make sure we're aware of this and notify ourselves to come and fix the problem.

A first, low-lift attempt was to put the workflow badges into the README so there's something bright red to tell us something broke. That's good if we're checking the Holonix repository but we aren't always looking at it.

Instead, it'd be good if we could post into MM or similar to ping ourselves on a workflow failure. Then we can react more quickly and keep on top of build failures.