nixos/xserver: fix evaluation of xkbDir

I solved it this way because it seems the current state-of-the-art 89e983786
The problem was introduced in #11930. Close #11951 (a different solution).

nixos/modules/services/x11/xserver.nix

@@ -281,10 +281,9 @@ in
       };
 
       xkbDir = mkOption {
-        type = types.str;
+        type = types.path;
-        default = "${pkgs.xkeyboard_config}/etc/X11/xkb";
         description = ''
-        Path used for -xkbdir xserver parameter.
+          Path used for -xkbdir xserver parameter.
         '';
       };
 
@@ -548,6 +547,8 @@ in
         xorg.xf86inputevdev
       ];
 
+    services.xserver.xkbDir = mkDefault "${pkgs.xkeyboard_config}/etc/X11/xkb";
+
     services.xserver.config =
       ''
         Section "ServerFlags"