Add LUCI-TryBot-Result label

This label is a temporary duplicate of TryBot-Result used by LUCI until
LUCI becomes default (at which point LUCI will set TryBot-Result
directly).

Change-Id: I14de652f24683e69846257bbd239796a2637e406
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/512675
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>