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

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://golang.org/cl/174600043

dashboard/app/cron.yaml

@@ -2,4 +2,5 @@ cron:
 - description: updates noise level for benchmarking results
   url: /perflearn?update=1
   schedule: every 24 hours
+  target: build