Send slack notification if nightly build failed

[teodanciu]

Description

This will send a notification on #ledger-internal Slack if the nightly build fails.

For this to work, we need another entry added to github settings -> Security -> Secrets and variables -> Actions -> Repository Secrets , namely : SLACK_WEBHOOK_URL
The value for this should be set to the Webhook URL of the app

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[JaredCorduan]

thank you @teodanciu !

[lehins]

👍

[teodanciu]

@JaredCorduan maybe tomorrow during the meeting we could add these settings to github together, so we can merge this?

[JaredCorduan]

@JaredCorduan maybe tomorrow during the meeting we could add these settings to github together, so we can merge this?

great idea!

[lehins]

SLACK_WEBHOOK_URL is now set.