cmd/go: fix TestNoCache on Plan 9

CL 91097 added TestNoCache. However, this
test is failing on Plan 9 because the HOME
environment variable doesn't contain the
home directory where the Go cache is located.

This change fixes the TestNoCache test
by using the home environment variable
instead of HOME on Plan 9.

Fixes #23644.

Change-Id: Icfb7a7a4c2852f159c93032b4081411628a2787f
Reviewed-on: https://go-review.googlesource.com/91216


Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>