Fix using ephemeral disks for /tmp etc. in EC2 instances

This code in amazon-image.nix:

  if mountFS "$device" "$mp" "" auto; then
    if [ -z "$diskForUnionfs" ]; then diskForUnionfs="$mp"; fi
  fi

relies on mountFS to return a zero exit status if mounting
succeeds. But the lustrateRoot check in mountFS was causing a non-zero
exit status. As a result /disk0 would be mounted, but not used for
/tmp.

(cherry picked from commit d082ed8c35dec48aee2afd1303b3c8b2a1b242b0)