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://golang.org/cl/174600043

cw2018/tools,cmars/tools,glasser/golang-tools,glasser/golang-tools,dmitris/tools,josharian/tools,springning/tools,syreclabs/go-tools,jdhenke/tools,jdhenke/tools,nicot/tools,magiccao/tools,runcom/tools,argandas/tools,cw2018/tools,sshetty10/tools,sshetty10/tools,4ad/tools,dmitris/tools,dmitris/tools,nicot/tools,maddyonline/tools,glasser/golang-tools,nicot/tools,syreclabs/go-tools,maddyonline/tools,argandas/tools,jdhenke/tools,matryer/tools,golang/tools,glasser/golang-tools,matryer/tools,springning/tools,cmars/tools,springning/tools,magiccao/tools,maddyonline/tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,runcom/tools,runcom/tools,maddyonline/tools,matryer/tools,springning/tools,cw2018/tools,sshetty10/tools,syreclabs/go-tools,magiccao/tools,syreclabs/go-tools,cw2018/tools,josharian/tools,runcom/tools,nicot/tools,josharian/tools,argandas/tools,4ad/tools,jdhenke/tools,golang/tools,cmars/tools,sshetty10/tools,matryer/tools,cmars/tools,4ad/tools,josharian/tools,dmitris/tools,golang/tools,magiccao/tools,argandas/tools