cmd: use testenv.Executable helper

cmd: use testenv.Executable helper

Change-Id: I25ac0e8d25d760bfde3bb7700f0feaa23f3e8ab1
Reviewed-on: https://go-review.googlesource.com/c/go/+/609302
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>