nixos/display-managers: add sessionData.desktops to XDG_DATA_DIRS

Fixes #100108

Alternative to https://github.com/NixOS/nixpkgs/pull/100112 which doesn't break stuff.

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

@@ -474,6 +474,12 @@ in
             )
             [dms wms]
           );
+
+    # Make xsessions and wayland sessions available in XDG_DATA_DIRS
+    # as some programs have behavior that depends on them being present
+    environment.sessionVariables.XDG_DATA_DIRS = [
+      "${cfg.displayManager.sessionData.desktops}/share"
+    ];
   };
 
   imports = [