Add a github-actions step to e-mail the team

For scheduled jobs, Github notifies the person who last touched the cron schedule ([documentation](https://docs.github.com/en/actions/guides/about-continuous-integration#notifications-for-workflow-runs)).

This is not particularly helpful, since the whole team needs to know when the `main` branch has failing tests. We should set up a dedicated e-mail step in unit/integration test workflows to send e-mails to the whole team.