(cherry picked from commit a13064a)

Motivation for this change

Fixes loading portals defined in xdg.portal.extraPortals from xdg-desktop-portal.service.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
  Verified fix on sway with:

  xdg.portal = {
    enable = true;
    extraPortals = with pkgs; [ xdg-desktop-portal-wlr ];
  };

• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.