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

tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go