Backport of CI: send slack notification on failed nightly job into release/1.15.x#18581

hc-github-team-consul-core wants to merge 1 commit intorelease/1.15.xhashicorp/consul:release/1.15.xfrom

backport/nightly-slack-notification/legally-smart-terrierhashicorp/consul:backport/nightly-slack-notification/legally-smart-terrierCopy head branch name to clipboard