path/filepath: fix TestWinSplitListTestsAreValid on some systems

path/filepath: fix TestWinSplitListTestsAreValid on some systems

The environment variables used in those tests override the default
OS ones. However, one of them (SystemRoot) seems to be required on
some Windows systems for invoking cmd.exe properly.

This fixes #4930 and #6568.

Change-Id: I23dfb67c1de86020711a3b59513f6adcbba12561
Reviewed-on: https://go-review.googlesource.com/36873
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>