cmd/go: keep WINDIR during TestScript

TestScript executes PowerShell. And PowerShell appears to require
WINDIR environment variable to exists on windows-arm.

So keep WINDIR environment variable when running PowerShell.

Fixes #30711

Change-Id: I14dde6614347325a8f3caf994ea8ab05294450a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/168859


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>