Instead of calling run synchronously, we pass it through bgrun
and immediately wait for it to finish. This pushes all jobs
to execute through the bgwork channel and therefore causes
them to exit cleanly in case of a compiler error.

Fixes #25981

Change-Id: I789a85d23fabf32d144ab85a3c9f53546cb7765a
Reviewed-on: https://go-review.googlesource.com/127776
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>