Certs do not verify when running on windows

When using kbld on windows, it doesn't use the system certs:

This works without having to pass in a cert:
```
docker pull some-registry.io/image
```

But this fails:
```
./kbld-windows.exe -f images.yaml
Get https://some-registry.io/v2/: x509: certificate signed by unknown authority
```

Using `--registry-verify-certs false` fails with the same error.

We think this is because of:
https://github.com/golang/go/issues/16736

Note:
We checked and if you do not set the `RootCAs` [here](https://github.com/vmware-tanzu/carvel-kbld/blob/develop/pkg/kbld/registry/registry.go#L186), it will work