misc/nacl: exclude cmd/link from the test zip.
misc/nacl: exclude cmd/link from the test zip.

It does not appear to be necessary, and cmd/link does not appear in release branches.

LGTM=rsc
R=adg, rsc
CC=golang-codereviews
https://codereview.appspot.com/176900044

firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old