[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