In the past, changes to GOPACKAGESDRIVER have led to some confusing
regtest failures. Explicitly set it off.
Updates golang/go#39384
Change-Id: I303a58380a5e46e6621c19b2edc40d43199bb343
Reviewed-on: https://go-review.googlesource.com/c/tools/+/240058
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>