title: How to update ssh configuration files in NixOs
body: <p>I've been playing with NixOs and was wondering how one can modify a config file.
  For example I tried to modify the ssh configuration file <code>/etc/ssh/ssh_config</code> (to allow root ) but
  I can't save the file, as it's only a link.
  I understand that modifying a file manually is not the way to do (because the system needs
  be aware of the modification), so what is the NixOs way to do it ? </p>