dashboard/coordinator: notify dashboard when we're building
dashboard/coordinator: notify dashboard when we're building

Fixes golang/go#9494

Change-Id: Ib9451d41b2dd02d30d2efddbc0ec410a34bdb42c
Reviewed-on: https://go-review.googlesource.com/2583
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

evandbrown/build,evandbrown/build,golang/build,golang/build,golang/build,proppy/build,proppy/build,proppy/build,golang/build,evandbrown/build,proppy/build