os: handle TMPDIR in TempDir on Plan 9

CL 129063 added a test in TestScript/mod_enabled,
which was failing on Plan 9.

The test was failing because the Init function
of the cmd/go/internal/modload package was
expecting ModRoot to be part of os.TempDir.

However, ModRoot was set to TMPDIR, while
os.TempDir is returning /tmp on Plan 9.

This change fixes the implementation of
os.TempDir on Plan 9 to handle the TMPDIR
environment variable, similarly to Unix.

Fixes #27065.

Change-Id: Id6ff926c5c379f63cab2dfc378fa6c15293fd453
Reviewed-on: https://go-review.googlesource.com/129775


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>