[milo] Disable GetLogs for buildbucket builds.
[milo] Disable GetLogs for buildbucket builds.

This is only used for the swarming-text-annotations deprecated path,
which doesn't flow through buildbucket.

R=2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org, jchinlee@chromium.org

Bug:
Change-Id: I69e45a43cf8b0f17220ff83f83ebcf0cd1f72b05
Reviewed-on: https://chromium-review.googlesource.com/750236
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

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