base.Cwd should be used instead.

Change-Id: I3dbdecf745b0823160984cc942c883dc04c91d7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/203037
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>