cmd: use internal/syscall/windows to get Windows version
internal/syscall/windows already provides a function to get the Windows
version. There is no need to use golang.org/x/sys/windows for this.
Change-Id: If31e9c662b10716ed6c3e9054604366e494345cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/622815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Carlos Amedee <carlos@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>