cmd/coordinator: remove buildStatus from most of benchmarks.go

This refactoring is in preparation for moving benchmarks.go to a
separate package.

Updates golang/go#19871

Change-Id: I2b30bf5416937e52b603aec8102131fdccceee42
Reviewed-on: https://go-review.googlesource.com/44173
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed