cmd/go: fix mod_gomodcache and generate_env TestScripts on Plan 9

Adjust these two tests for Plan 9,
which uses $path instead of $PATH,
and $home instead of $HOME.

Fixes #39599

Change-Id: Idba95e07d307e76f0f61acd904905b417c52d43a
Reviewed-on: https://go-review.googlesource.com/c/go/+/237941


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>