misc/dashboard/builder: make -fail mode faster
misc/dashboard/builder: make -fail mode faster

Don't check out the repository if we're just going to fail everything.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7453048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go