cmd/coordinator: ignore ssacheck failures for old commits
cmd/coordinator: ignore ssacheck failures for old commits

If a commit doesn't have the new cmd/dist -compile-only flag, just skip
the ssacheck tests.

Change-Id: I41a5ce39968a073cd4be97e31bd26ffc5cf68699
Reviewed-on: https://go-review.googlesource.com/22793
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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