Milo: Buildbot build grpc endpoint fix
Milo: Buildbot build grpc endpoint fix

Previous CL accidentally pointed to the endpoint that returned a Milo build (A
generic build datastructure meant for both Luci and buildbot builds) instead of
the buildbot build.

BUG=642461

Review-Url: https://codereview.chromium.org/2360333003

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