It only cancel previous runs on forks or for pull request builds
and does not run for zulip/zulip repo pushes. It finishes pretty
quickly (under 1 minute) so it fine to have it as a workflow
rather than to add a new step under every single job to cancel it's
previous runs.
The only downside to this is that GitHub creates a notification for
the cancelled job just like it does for failed jobs!!!
Anders Kaseorg
Priyank Patel