cmd/compile/internal/logopt: preserve env while running command

The test was not preserving temporary directory flags leading to a
failure on windows with:

    mkdir C:\WINDOWS\go-build315158903: Access is denied.

Fixes #38251

Change-Id: I6ee31b31e84b7f6e75ea6ee0f3b8c094835bf5d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/227497


Reviewed-by: David Chase <drchase@google.com>