cmd/go: set GOROOT when testing "go install cmd/fix"
cmd/go: set GOROOT when testing "go install cmd/fix"

This particular test would never pass unless you had GOROOT set in your
environment. This changes makes the test use the baked-in GOROOT, as it
does with GOOS and GOARCH.

R=golang-dev, dave, iant
CC=golang-dev
https://codereview.appspot.com/43080043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang