cmd/coordinator: on trybot failure, mention that trybots don't rebase
cmd/coordinator: on trybot failure, mention that trybots don't rebase

Updates golang/go#19664
Updates golang/go#9858
Updates golang/go#30807

Change-Id: I378979e2745b4d34286a3a038161eb03148771e7
Reviewed-on: https://go-review.googlesource.com/c/build/+/170457
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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