[bbagent] Use buildbucket redirect URL.
[bbagent] Use buildbucket redirect URL.

R=ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org, tandrii@chromium.org

Bug: 910369
Change-Id: Icd95fa819a70d064bd2d01bcdf260a04c1cff837
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1838134
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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