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>

src/cmd/go/script_test.go

@@ -84,6 +84,7 @@ type backgroundCmd struct {
 
 var extraEnvKeys = []string{
 	"SYSTEMROOT",         // must be preserved on Windows to find DLLs; golang.org/issue/25210
+	"WINDIR",             // must be preserved on Windows to be able to run PowerShell command; golang.org/issue/30711
 	"LD_LIBRARY_PATH",    // must be preserved on Unix systems to find shared libraries
 	"CC",                 // don't lose user settings when invoking cgo
 	"GO_TESTING_GOTOOLS", // for gccgo testing

src/cmd/go/testdata/script/build_acl_windows.txt

 [!windows] stop
-[arm] skip # TODO(golang.org/issue/30711): Skip broken test.
 [!exec:icacls] skip
 [!exec:powershell] skip