This should call unix.Exec, not syscall.Exec. Thanks Tobias Klauser
for the spot.

Change-Id: Iddae390891a66652b071e05c1a5a25bbfdb19e52
Reviewed-on: https://go-review.googlesource.com/101435
Run-TryBot: Kevin Burke <kev@inburke.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>