nixos/sddm: Use libsForQt514.sddm if needed (for lxqt)

Currently lxqt is a desktop environment that's compiled against qt514.
To avoid possible issues (#101369), we (hopefully) use the same qt
version as the desktop environment at hand. LXQT should move to qt515,
and for the long term the correct qt version should be inherited by the
sddm module.

nixos/modules/services/x11/display-managers/sddm.nix

@@ -9,7 +9,12 @@ let
   cfg = dmcfg.sddm;
   xEnv = config.systemd.services.display-manager.environment;
 
-  inherit (pkgs) sddm;
+  sddm = if config.services.xserver.desktopManager.lxqt.enable then
+    # TODO: Move lxqt to libsForQt515
+    pkgs.libsForQt514.sddm
+  else
+    pkgs.libsForQt5.sddm
+  ;
 
   xserverWrapper = pkgs.writeScript "xserver-wrapper" ''
     #!/bin/sh

pkgs/top-level/all-packages.nix

@@ -25901,7 +25901,8 @@ in
   lumina = recurseIntoAttrs (callPackage ../desktops/lumina { });
 
   lxqt = recurseIntoAttrs (import ../desktops/lxqt {
-    # TODO: Update these to qt515 at some point
+    # TODO: Update these to qt515 at some point. When doing it, please remove
+    # the choice of libsForQt5*.sddm in sddm's module.
     qt5 = qt514;
     libsForQt5 = libsForQt514;
     inherit pkgs;