nixos-enter: Don't require root

Of course, you'll get a bunch of warnings from the activation script:

  $ nixos-enter --root /tmp/mnt/
  setting up /etc...
  mount: /dev: permission denied.
  mount: /dev/pts: permission denied.
  mount: /dev/shm: permission denied.
  mount: /sys: permission denied.
  /nix/var/nix/profiles/system/activate: line 74: /proc/sys/kernel/modprobe: Permission denied
  chown: changing ownership of '/run/wrappers/wrappers.0pKlU8JsvV/dbus-daemon-launch-helper': Invalid argument
  NOTE: Under Linux, effective file capabilities must either be empty, or
        exactly match the union of selected permitted and inheritable bits.
  Failed to set capabilities on file `/run/wrappers/wrappers.0pKlU8JsvV/ping' (Operation not permitted)
  chown: changing ownership of '/run/wrappers/wrappers.0pKlU8JsvV/unix_chkpwd': Invalid argument

  [root@nixos:/]#