cmd/coordinator: enable staging builds by default
cmd/coordinator: enable staging builds by default

Change-Id: If796b1df593620f17d6cb96f2aa89f5c9a538338
Reviewed-on: https://go-review.googlesource.com/38307
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build