Use mkosi.key/mkosi.crt for SSH authentication

Use mkosi.key/mkosi.crt for SSH authentication

Instead of using the user's SSH certificate and key, let's use the
X509 certificate and private key generated by 'mkosi genkey' instead.

This saves us from having to rely on ssh-agent to get the public key
or doing otherwise complicated logic to try and find the public and
private key. We also avoid always needing a separate public/private
key just for SSH by automatically converting the X509 certificate to
a SSH public key.