windows: build env_windows_test.go only go Go 1.21 and above

This test imports the "slices" package, which did not exist in Go 1.20.
The test passes on Go 1.21 and above, and the behavior of the function
under test is unlikely to vary by platform, so it doesn't seem worth
refactoring the test to work with older releases.

Updates golang/go#65055.
Fixes golang/go#65223.

Change-Id: I5f32106d6057b779579a87750633bc57f97fe152
Cq-Include-Trybots: luci.golang.try:x_sys-go1.20-windows-386,x_sys-go1.20-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/sys/+/557975
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

windows/env_windows_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.21
+
 package windows_test
 
 import (