mkvbox: implicitly write to stdout

Writing directly to `/dev/stdout` doesn't work when `sudo` is involved:

```
alice$ sudo -u bob -i
bob$ echo foo > /dev/stdout
bash: /dev/stdout: Permission denied
bob$ ls -Ll /dev/stdout
crw--w---- 1 alice tty  136, 12 Dec 11 16:52 /dev/stdout
```