qt5.qtwebengine: fix Qt version number in cmake

Previously fixed in 97904f8424ccbba6de4ae63e7e97c2c4edf65567
and #118084, but the fix was only applied to linux platforms.

This fixes libraries that depend on qtwebengine under darwin platforms.

pkgs/development/libraries/qt-5/modules/qtwebengine.nix

···
     Prefix = ..
     EOF
 
+  '' + ''
     # Fix for out-of-sync QtWebEngine and Qt releases (since 5.15.3)
     sed 's/${lib.head (lib.splitString "-" version)} /${qtCompatVersion} /' -i "$out"/lib/cmake/*/*Config.cmake
   '';