This is part one of two changes intended to make it easier to debug builder failures.

runOutput allows us to control the io.Writer passed to a subcommand. The intention is to add additional debugging information before and after the build which will then be capture and sent to the dashboard.

In this proposal, the only additional information is the build status. See http://build.golang.org/log/e7b5bf435b4de1913fc61781b3295fb3f03aeb6e

R=adg
CC=golang-dev
https://golang.org/cl/7303090