Default `auth_key_file` is insecure

[nbraud]

A process running as a given user can (in most cases) modify ~/.ssh/authorized_keys, for instance to add a new key they control before invoking whatever is using pam_rssh, subverting the authentication it is supposed to provide.

NixOS/nixpkgs#31611 provides an example (ab)using this to bypass pam_ssh_agent_auth in the context of sudo, which I believe would work similarly against pam_rssh.