Chat notification when DCAR main build is broken

We currently don't have an automated way of being alerted when DCAR's main build is broken.

For example this happened on this main commit:
https://github.com/guardian/dotcom-rendering/commit/ae0a2ed034ac832a8471951c32b1fc934462fe27

It would be useful to receive a chat notification on the public WebX channel when this happens.

We need to investigate the best method to do this, it could be in the existing build check workflow or a new check.

https://github.com/guardian/dotcom-rendering/actions/runs/18372299100/job/52338144415

https://github.com/guardian/dotcom-rendering/blob/2482041ce59184f2e4cb9a4d9736412d32997ede/.github/workflows/build-check.yml