If GOBIN is not empty the build moves the go executable
to a new path. When this test runs it fails to find the
go cmd in the GOROOT.

Change-Id: I100def0fbcb9691b13776f795b1d1725e36d8102
Reviewed-on: https://go-review.googlesource.com/8735Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>