cmd/coordinator: blacklist some builder/repo pairs

These never work and are a waste of resources and unnecessary scary
red on the page.

Change-Id: I2d8f8908096dee01618129a9082e98f3ffd92bb7
Reviewed-on: https://go-review.googlesource.com/37467
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>