Revert "ci: only trigger slack webhook from canary" (#69648)
Reverts vercel/next.js#69632
It appears to not be working quite right.
skip: https://github.com/vercel/next.js/actions/runs/10692363199
failed retry:
https://github.com/vercel/next.js/actions/runs/10692293873/job/29641598903
