nixos/systemd: provide libidn2 for systemd-resolved

systemd started using dlopen() for some of their "optional"
dependencies.

Apparently, `libidn2` isn't so optional, and systemd-resolved doesn't
work without libidn2 present, breaking DNS resolution.

Fixes https://github.com/NixOS/nixpkgs/issues/107537

Upstream bug: https://github.com/systemd/systemd/issues/18078

nixos/modules/system/boot/resolved.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, pkgs, lib, ... }:
 
 with lib;
 let
@@ -150,6 +150,9 @@ in
       wantedBy = [ "multi-user.target" ];
       aliases = [ "dbus-org.freedesktop.resolve1.service" ];
       restartTriggers = [ config.environment.etc."systemd/resolved.conf".source ];
+      # Upstream bug: https://github.com/systemd/systemd/issues/18078
+      # systemd-resolved without libidn2 is broken
+      environment.LD_LIBRARY_PATH = "${lib.getLib pkgs.libidn2}/lib";
     };
 
     environment.etc = {