Try enabling Slack notifications if Checks fail

[guarani]

Slack notifications were not previously enabled when the check job failed. This PR enables Slack notifications and triggers them if the "Check Correctness" job fails.

To test:

TO-DO

PR submission checklist:

• [x] I have considered adding unit tests where possible.
• [x] I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.