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>

env/netbsd-amd64/make.bash

@@ -15,9 +15,8 @@ set -e -x
 
 ANITA_VERSION=1.39
 ARCH=amd64
-# Actually, the release is 7.99.26 (-current) but this is what the packages
-# have been built for.
-RELEASE=7.0
+# The release that the packages have been built for.
+RELEASE=7.0_2016Q4
 
 # Must use GNU tar. On NetBSD, tar is BSD tar and gtar is GNU.
 TAR=tar

env/netbsd-amd64/mkvm.py

@@ -56,9 +56,9 @@ def find_latest_release(arch):
 !route add default \`ifconfig vioif0 | awk '/inet / { print \$2 }' | sed 's/[0-9]*$/1/'\` -ifp vioif0
 EOF""",
     "dhcpcd",
-    "env PKG_PATH=https://cdn.netbsd.org/pub/pkgsrc/packages/NetBSD/%s/%s/All/ pkg_add bash curl dhcpcd" % (arch, release),
-    "env PKG_PATH=https://cdn.netbsd.org/pub/pkgsrc/packages/NetBSD/%s/%s/All/ pkg_add git-base" % (arch, release),
-    "env PKG_PATH=https://cdn.netbsd.org/pub/pkgsrc/packages/NetBSD/%s/%s/All/ pkg_add mozilla-rootcerts go14" % (arch, release),
+    "env PKG_PATH=https://ftp.netbsd.org/pub/pkgsrc/packages/NetBSD/%s/%s/All/ pkg_add bash curl dhcpcd" % (arch, release),
+    "env PKG_PATH=https://ftp.netbsd.org/pub/pkgsrc/packages/NetBSD/%s/%s/All/ pkg_add git-base" % (arch, release),
+    "env PKG_PATH=https://ftp.netbsd.org/pub/pkgsrc/packages/NetBSD/%s/%s/All/ pkg_add mozilla-rootcerts go14" % (arch, release),
     """ed /etc/fstab << EOF
 H
 %s/wd0/sd0/