Kubeval doesn't work correctly with xargs

`kubeval` incorrectly reads from stdin when used with `xargs`. Here's an example:

```
$ echo "foo.txt" > files.txt
$ xargs kubeval < files.txt
standard input appears to be an empty document
```

Here's a workaround: `xargs -I m sh -c 'kubeval m < /dev/stdin' < files.txt`.