qt6.qtwebengine: fix QtWebEngine build crash

QtWebEngine tries to build with clang 14, which links libc++ 14, but the
top-level libc++ on Dariwn is libc++ 16. This results in a crash during
the build, which is fixed by building with the default stdenv.

pkgs/development/libraries/qt-6/default.nix

···
       qtwebchannel = callPackage ./modules/qtwebchannel.nix { };
       qtwebengine = callPackage ./modules/qtwebengine.nix {
         inherit (darwin) bootstrap_cmds cctools xnu;
-        inherit (darwin.apple_sdk_11_0) libpm libunwind llvmPackages_14;
+        inherit (darwin.apple_sdk_11_0) libpm libunwind;
         inherit (darwin.apple_sdk_11_0.libs) sandbox;
         inherit (darwin.apple_sdk_11_0.frameworks)
           AGL AVFoundation Accelerate Cocoa CoreLocation CoreML ForceFeedback

pkgs/development/libraries/qt-6/modules/qtwebengine.nix

···
 , mesa
 , enableProprietaryCodecs ? true
   # darwin
-, llvmPackages_14
 , bootstrap_cmds
 , cctools
 , xcbuild
···
     gn
     nodejs
   ] ++ lib.optionals stdenv.isDarwin [
-    llvmPackages_14.clang
     bootstrap_cmds
     cctools
     xcbuild