Removes a potential data race between os.Setenv and runtime.GOROOT,
along with a bug where os.Setenv would only sometimes change the
value of runtime.GOROOT.

Change-Id: I7d2a905115c667ea6e73f349f3784a1d3e8f810d
Reviewed-on: https://go-review.googlesource.com/6611Reviewed-by: Keith Randall <khr@golang.org>