[release-branch.go1.3] nacltest.bash: unset GOROOT

The GOROOT baked into the go command being run is correct.
The GOROOT in the environment may not be. Remove it.

Fixes nacltest.bash if you have GOROOT set elsewhere.

This is already fixed in the default branch.

LGTM=r
R=golang-codereviews, r
CC=bradfitz, golang-codereviews, iant
https://golang.org/cl/147380044

src/nacltest.bash

@@ -45,6 +45,7 @@ if [ ! -f make.bash ]; then
 	exit 1
 fi
 GOOS=$GOHOSTOS GOARCH=$GOHOSTARCH ./make.bash
+unset GOROOT
 
 # Build zip file embedded in package syscall.
 gobin=${GOBIN:-$(pwd)/../bin}