cmd/coordinator: implement trybot set cancellation
cmd/coordinator: implement trybot set cancellation

Fixes golang/go#35701

Change-Id: I92f7818c64cc49675f1f0a2f74533ba9558a84fd
Reviewed-on: https://go-review.googlesource.com/c/build/+/207842
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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