env/netbsd-amd64: fix networking on GCE
env/netbsd-amd64: fix networking on GCE

NetBSD-8 builds after 2017-09-15 have a fix for the previous
routing issue on Google Compute Engine.

I have tested an image created with this script, and it successfully
runs a buildlet.

Change-Id: I02dc7155e43b6e0b18c83601b36cb290761d5dc9
Reviewed-on: https://go-review.googlesource.com/64750
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/build,golang/build,golang/build,golang/build