https://golang.org/cl/52390 was submitted too early with failing trybots.
This fixes it, hiding the cloud.google.com stuff behind a build tag,
used by the Dockerfile but not the Go build system.
Change-Id: I66c6b40d4b06bf6c763f3ab221c7997856bfc910
Reviewed-on: https://go-review.googlesource.com/52470
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jessie Frazelle <me@jessfraz.com>
Brad Fitzpatrick