[tech] add CI failure Slack notifications for master branch
Created by: adipasquale
this will notify us on slack when a job's build/test/lint step fails on the master branch only.
cf https://circleci.com/orbs/registry/orb/circleci/slack
I added the SLACK_WEBHOOK
env var on CircleCI
I tested once by switching to this branch and failing a test on purpose, it seemd to work