cmd/coordinator: favor trybot work over post-submit work on process start
cmd/coordinator: favor trybot work over post-submit work on process start

Updates golang/go#19178

Change-Id: I054c852db398a9474e53254c42eb5f77c44fe348
Reviewed-on: https://go-review.googlesource.com/c/build/+/207464
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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