cmd/dist: don't print misleading warning when using GOROOT_FINAL
cmd/dist: don't print misleading warning when using GOROOT_FINAL

Fixes issue 5240.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13546044

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