qt6Packages.stdenv: condition on `config.allowAliases` (#370783)

pkgs/top-level/all-packages.nix

···
   qt6 = recurseIntoAttrs (callPackage ../development/libraries/qt-6 { });
 
   qt6Packages = recurseIntoAttrs (import ./qt6-packages.nix {
-    inherit lib __splicedPackages makeScopeWithSplicing' generateSplicesForMkScope pkgsHostTarget kdePackages;
+    inherit lib config __splicedPackages makeScopeWithSplicing' generateSplicesForMkScope pkgsHostTarget kdePackages;
     inherit stdenv;
   });
 

pkgs/top-level/qt6-packages.nix

···
 # this file.
 
 { lib
+, config
 , __splicedPackages
 , makeScopeWithSplicing'
 , generateSplicesForMkScope
···
 
   xwaylandvideobridge = kdePackages.callPackage ../tools/wayland/xwaylandvideobridge { };
   });
-} // {
+} // lib.optionalAttrs config.allowAliases {
   stdenv = lib.warn "qt6Packages.stdenv is deprecated. Use stdenv instead." stdenv; # Added for 25.05
 }