The packages.Load call first selects a driver to use. In this case it
is invariably going to be the go list command, since GOPACKAGESDRIVER
is being set to off in Config.Env.

In some environments, such as openbsd/amd64, the go command might not
be able to run properly if the PATH environment variable isn't passed
on to it. There may be room for improvement there, but that's outside
of the scope of this test.

Fixes golang/go#79907.

Change-Id: I6734713f0b2670007078d990325797b075f6f8fc
Cq-Include-Trybots: luci.golang.try:x_tools-gotip-openbsd-amd64
Reviewed-on: https://go-review.googlesource.com/c/tools/+/789500
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>