qt6: prevent Xcode version sniffing (#421405)

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

···
     ++ lib.optionals stdenv.hostPlatform.isDarwin [
       "-DQT_FEATURE_rpath=OFF"
       "-DQT_NO_XCODE_MIN_VERSION_CHECK=ON"
+      # This is only used for the min version check, which we disabled above.
+      # When this variable is not set, cmake tries to execute xcodebuild
+      # to query the version.
+      "-DQT_INTERNAL_XCODE_VERSION=0.1"
     ]
     ++ lib.optionals isCrossBuild [
       "-DQT_HOST_PATH=${pkgsBuildBuild.qt6.qtbase}"

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

···
       # don't leak OS version into the final output
       # https://bugreports.qt.io/browse/QTBUG-136060
       [ "-DCMAKE_SYSTEM_VERSION=" ]
-      ++ lib.optional stdenv.hostPlatform.isDarwin "-DQT_NO_XCODE_MIN_VERSION_CHECK=ON"
+      ++ lib.optionals stdenv.hostPlatform.isDarwin [
+        "-DQT_NO_XCODE_MIN_VERSION_CHECK=ON"
+        # This is only used for the min version check, which we disabled above.
+        # When this variable is not set, cmake tries to execute xcodebuild
+        # to query the version.
+        "-DQT_INTERNAL_XCODE_VERSION=0.1"
+      ]
       ++ args.cmakeFlags or [ ];
 
     moveToDev = false;