Ok, I've learned how to do this and manually failed those builds now. It should be safe to keep the dragonfly builder running now. Please let me know if there's anything else.

For posterity, I used the approach @bradfitz recommended, which was to hit the /result endpoint of the dashboard (https://build.golang.org) manually with a one-off Go command; I'll clean it up and upstream it if this needs to be done more often. It was based on this code in cmd/coordinator. That endpoint requires a builder key, which I have access to.

Ideally, we should resolve the underlying issue and make it so that the builder has a timeout and doesn't hang forever on some bad commits.