cmd/coordinator: don't build pre-Go 1.7 code on Sierra builders

Updates golang/go#18751

Change-Id: Iadd7dded079376a9bf9717ce8071604cee95d8ef
Reviewed-on: https://go-review.googlesource.com/35643
Reviewed-by: Kevin Burke <kev@inburke.com>
Reviewed-by: Russ Cox <rsc@golang.org>
2 files changed