Mirror the `-i` option from SSH, so you can run `nixos-anywhere ~/.ssh/other_key`.
This commit also fixes an issue where the generated key-pair would stay
around when using the SSH_PRIVATE_KEY env var.
this allows passing multiple disk encryption keys, some of which
might come in the form of unix pipes.
It can be used with bash file substition facilities to pass a secret
to the remote machine without writing it locally to disk.
Example
--disk-encryption-keys /tmp/disk-1.key <(echo "my-secret")
--disk-encryption-keys /tmp/disk-2.key /tmp/static-file.key
