build: Use stable branch packages for netbsd-amd64.

In the last iteration, Brad got an inconsistent (i.e. non-working) set
of packages because (a) the CDN uses different mirrors, some of which
are more stale than others and (b) it was using bleeding-edge packages.

Switch to a hardcoded mirror (the main one) and stable (2016Q4)
packages.

Change-Id: I45e5ec8596b522c6284ff4b8f08edd1a29f5597b
Reviewed-on: https://go-review.googlesource.com/37190
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed