[buildbucket] proxy 100% dev traffic for UpdateBuild() to GoBB
[buildbucket] proxy 100% dev traffic for UpdateBuild() to GoBB

Bug:1110990
Change-Id: I4a9e142de6b6db600e0566c813359c168fbd76b2
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2688445
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go