x/build/cmd/coordinator/buildongce: use the latest CoreOS image

Also remove the public key argument.

Fixes golang/go#14111

Change-Id: I18c603f6f393075e69b766835961abbe7c17369c
Reviewed-on: https://go-review.googlesource.com/18957
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed