xorg.xorgserver: wrap Xvfb with required flags

nixos/modules/services/x11/terminal-server.nix

···
 
 with lib;
 
-let
-
-  # Wrap Xvfb to set some flags/variables.
-  xvfbWrapper = pkgs.writeScriptBin "Xvfb"
-    ''
-      #! ${pkgs.stdenv.shell}
-      export XKB_BINDIR=${pkgs.xorg.xkbcomp}/bin
-      export XORG_DRI_DRIVER_PATH=${pkgs.mesa}/lib/dri
-      exec ${pkgs.xorg.xorgserver}/bin/Xvfb "$@" -xkbdir ${pkgs.xkeyboard_config}/etc/X11/xkb
-    '';
-
-in
-
 {
 
   config = {
···
       { description = "Terminal Server";
 
         path =
-          [ xvfbWrapper pkgs.gawk pkgs.which pkgs.openssl pkgs.xorg.xauth
+          [ pkgs.xorgserver pkgs.gawk pkgs.which pkgs.openssl pkgs.xorg.xauth
             pkgs.nettools pkgs.shadow pkgs.procps pkgs.utillinux pkgs.bash
           ];
 

pkgs/servers/x11/xorg/overrides.nix

···
           wrapProgram $out/bin/Xephyr \
             --set XKB_BINDIR "${xorg.xkbcomp}/bin" \
             --add-flags "-xkbdir ${xorg.xkeyboardconfig}/share/X11/xkb"
+          wrapProgram $out/bin/Xvfb \
+            --set XKB_BINDIR "${xorg.xkbcomp}/bin" \
+            --set XORG_DRI_DRIVER_PATH ${args.mesa}/lib/dri \
+            --add-flags "-xkbdir ${xorg.xkeyboardconfig}/share/X11/xkb"
         '';
         passthru.version = version; # needed by virtualbox guest additions
       } else {

pkgs/tools/X11/xpra/default.nix

···
     pillow pygtk pygobject
   ];
 
-  postPatch = ''
-    sed -i 's|DEFAULT_XVFB_COMMAND = "Xvfb|DEFAULT_XVFB_COMMAND = "Xvfb -xkbdir ${xkeyboard_config}/etc/X11/xkb|' xpra/platform/features.py
-  '';
-
   preBuild = ''
     export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE $(pkg-config --cflags gtk+-2.0) $(pkg-config --cflags pygtk-2.0) $(pkg-config --cflags xtst)"
   '';

pkgs/tools/X11/xpra/gtk3.nix

···
     pygobject3 pycairo cython
   ];
 
-  postPatch = ''
-    sed -i 's|DEFAULT_XVFB_COMMAND = "Xvfb|DEFAULT_XVFB_COMMAND = "Xvfb -xkbdir ${xkeyboard_config}/etc/X11/xkb|' xpra/platform/features.py
-  '';
-
   preBuild = ''
     export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE $(pkg-config --cflags gtk+-3.0) $(pkg-config --cflags xtst)"
   '';