nix-daemon: source nix-profile-daemon.sh only once

On my system (Ubuntu 24.04 with nix installed using
https://zero-to-nix.com/), I noticed that my PATH
contained multiple times the following entries:

  /home/thomas/.nix-profile/bin
  /nix/var/nix/profiles/default/bin

Fix it by inserting a missing `export`, to make
sure `nix-daemon.sh` is really only executed once.

Author: thomie

scripts/nix-profile-daemon.sh.in

@@ -1,7 +1,7 @@
 # Only execute this file once per shell.
 # This file is tested by tests/installer/default.nix.
 if [ -n "${__ETC_PROFILE_NIX_SOURCED:-}" ]; then return; fi
-__ETC_PROFILE_NIX_SOURCED=1
+export __ETC_PROFILE_NIX_SOURCED=1
 
 NIX_LINK=$HOME/.nix-profile
 if [ -n "${XDG_STATE_HOME-}" ]; then