feat: get slack notif in case of spec build failure

[letypequividelespoubelles]

Note

Adds Slack failure/cancellation notifications to the spec compile workflow and pins actions/checkout to v5.

• CI (GitHub Actions):
  • Failure notifications: Add Slack notification step on failure() or cancelled() using slackapi/slack-github-action with webhook payload.
  • Hard pin: Update actions/checkout to v5 via commit SHA in .github/workflows/compile.yml.

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