Use internal/testenv package to get the right go binary.
Otherwise, I think we're just grabbing an old one from the environment.

Fixes #22560.

Change-Id: Id5b743b24717e15ec8ffbcfae4dc3e5f6a87b9a9
Reviewed-on: https://go-review.googlesource.com/76090
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: David Chase <drchase@google.com>