[buildbucket] send 100% of UpdateBuild reqs to default-go
[buildbucket] send 100% of UpdateBuild reqs to default-go

Bug: 1110990
Change-Id: I989f1260ccc2e2b6b0c9efc353adf299d3ae638a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2704727
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

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