cmd/coordinator: don't run racecompile builder on old commits

We still run some of it (at least for now, it's harmless, and in its
own goroutine), but not the -c part.

Fixes golang/go#20222

Change-Id: I37a39a41c8a5f2ebaee70aeba390c58b22399d3d
Reviewed-on: https://go-review.googlesource.com/43619
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
2 files changed