Add a slack notification when the `nightly` or `nightly-next` scheduled builds fail

[leighmcculloch]

Add a slack notification when the nightly or nightly-next scheduled builds fail.

Include in the notification, if possible:

• A link to the failed run.
• The versions of all the software being built.
• Whether build failed, and which build, or test failed, and which test.

The notification should use the action cache to cache the fact the notification was sent so that repeated scheduled builds do not spam.