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)

nixos/modules/system/boot/stage-1-init.sh

@@ -318,6 +318,8 @@ mountFS() {
     [ "$mountPoint" == "/" ] &&
         [ -f "/mnt-root/etc/NIXOS_LUSTRATE" ] &&
         lustrateRoot "/mnt-root"
+
+    true
 }
 
 lustrateRoot () {