nixos-enter: redirect to fd2 instead of a file named /dev/stderr

In some cases, /dev/stderr may not point to a sensible location. For
example, running nixos-enter inside a systemd unit where the unit's
StandardOutput and StandardError are set to be sockets. In these
cases, this line would fail.

Piping to fd2 directly works just as well, even under strange and
twisted executions.

Co-authored-by: Michael Bishop <michael.bishop@iohk.io>