xdg-mime-apps: add legacy mimeapps.list path

Although `.local/share/applications/mimeapps.list` is deprecated, this
file is still being read by some applications. To ensure compatibility
duplicate the file as recommended in
https://wiki.archlinux.org/index.php/XDG_MIME_Applications#mimeapps.list

modules/misc/xdg-mime-apps.nix

@@ -75,6 +75,10 @@ in
   };
 
   config = mkIf cfg.enable {
+    # Deprecated but still used by some applications.
+    home.file.".local/share/applications/mimeapps.list".source =
+      config.xdg.configFile."mimeapps.list".source;
+
     xdg.configFile."mimeapps.list".text =
       let
         joinValues = mapAttrs (n: concatStringsSep ";");