[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