qtwebengine: replace targetPlatform with hostPlatform

stdenv.targetPlatform really shouldn't be used by software that
doesn't generate or manipulate binaries.

I did a mass-replacement in
#267229 and it affected eval
for only two packages; one of them is qt6-qtwebengine, but it is
affected only on Darwin.  So I am breaking this out into a separate
PR.

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

@@ -196,7 +196,7 @@ qtModule {
   ] ++ lib.optionals enableProprietaryCodecs [
     "-DQT_FEATURE_webengine_proprietary_codecs=ON"
   ] ++ lib.optionals stdenv.isDarwin [
-    "-DCMAKE_OSX_DEPLOYMENT_TARGET=${stdenv.targetPlatform.darwinSdkVersion}"
+    "-DCMAKE_OSX_DEPLOYMENT_TARGET=${stdenv.hostPlatform.darwinSdkVersion}"
   ];
 
   propagatedBuildInputs = [