Fix COOKIE issue by setting RELEASE_COOKIE environment variable

- Remove dummy COOKIE file creation (doesn't work in Nix store)
- Set RELEASE_COOKIE env var in wrapper script instead
- This follows NixOS best practices for Elixir releases

nix/package.nix

@@ -12,15 +12,13 @@ beamPackages.mixRelease rec {
   
   # Disable distributed Erlang to avoid COOKIE requirement
   postInstall = ''
-    # Create a dummy COOKIE file to satisfy the release boot script
-    echo "dummy_cookie" > $out/releases/COOKIE
-    
-    # Create wrapper script that sets proper environment
+    # Create wrapper script that sets proper environment including COOKIE
     mv $out/bin/systant $out/bin/.systant-wrapped
     cat > $out/bin/systant << EOF
 #!/bin/sh
 export RELEASE_DISTRIBUTION=none
 export RELEASE_NODE=nonode@nohost
+export RELEASE_COOKIE=dummy_cookie_for_single_node
 exec "$out/bin/.systant-wrapped" "\$@"
 EOF
     chmod +x $out/bin/systant