cmd/compilebench: use go command in goroot, not environment

The "go" command is a random "go" in the environment, not
necessarily the one under test.  Use the go command in the
goroot we're testing.

This CL removes the need to add $GOROOT/bin to your path
before running compilebench.

Change-Id: Ieb7f441f8287105e13446006e73b760d80e51e03
Reviewed-on: https://go-review.googlesource.com/42932
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed