dashboard/buildlet: set GOTESTONLY=std on plan9

This resulted in our first "ok" on the dashboard for Plan 9 with the
buildlet, in 19 minutes. It only runs the std tests, and nothing else
after that.

Update golang/go#9491

Change-Id: Iad77a594f83bfd3fa72596bcc3057645d9c9bb4c
Reviewed-on: https://go-review.googlesource.com/2523
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed