OSValidator doesn't work on Windows

It looks like this supports windows since there is a [windows spec](https://github.com/kubernetes/system-validators/blob/1e0d9c33b4d2294743bc03b6154a9538ad64e340/validators/types_windows.go#L28) but os validator fails: ``` Validating os... Validating kernel... KERNEL_VERSION: 10.0.22631 failed to get os name: exec: "uname": executable file not found in %PATH% ```