os/exec: preserve the process environment when invoking TestHelperProcess

os/exec: preserve the process environment when invoking TestHelperProcess

Also log errors from the lsof command on failure.
(That's how the missing environment was discovered.)

Updates #25628

Change-Id: I71594f60c15d0d254d5d4a86deec7431314c92ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/201717
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>