builders: let coordinator give dashboard a build status URL

I would like a way to POST to the dashboard an URL of an in-progress build.

Then it can be rendered as "..." on https://build.golang.org/ (instead of a blank cell) with the "..." being a link to the coordinator to show the build live as it happens. Or an animated spinny gopher gif. Whatever.

This in-progress-URL field would only be for display; it would not affect the other queries in an way.
