x/tools/dashboard/app: have cron.yaml point to build.golang.org
x/tools/dashboard/app: have cron.yaml point to build.golang.org

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/174600043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools