nixos-module: set RELEASE_COOKIE so the release starts

The Elixir release's start script reads releases/COOKIE which isn't baked
in, so the service crashed on boot (cat: releases/COOKIE: No such file).
Set RELEASE_COOKIE in the systemd Environment to fix startup.

nixos-module.nix

@@ -63,6 +63,9 @@ in
           "PORT=${toString cfg.port}"
           # Agents are managed at runtime via the admin API and persisted here.
           "AGENTS_FILE=/var/lib/n8n-openai/agents.json"
+          # Elixir release needs a cookie; the release's start script reads
+          # releases/COOKIE which isn't baked in, so set it explicitly.
+          "RELEASE_COOKIE=n8n-openai-adapter"
         ];
         Restart = "always";
         RestartSec = 5;