builder: use go-build.appspot.com instead of godashboard by default
builder: use go-build.appspot.com instead of godashboard by default

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5489097

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang