pv command is missing

`eos-write-image` works better when `pv` is available

```
$ sudo eos-write-image eos-eos6.0-amd64-amd64.251107-052648.base.iso  /dev/sda
Guessed that /dev/sda is a removable disk; pass --fixed if this is wrong.
Are you sure you want to overwrite all data on /dev/sda? [y/N] y
Proceeding with no pv, there will be no progress message!
Try 'sudo apt-get install pv'
Writing...
```