qt: fixate the linux version

When building on Linux, the Qt build process includes
`CMAKE_SYSTEM_VERSION` in the resulting module description, which
defaults to the kernel version of the build host, breaking
reproducibility.

This change clears the `CMAKE_SYSTEM_VERSION` which is supported
by the build process and leaves out the version, giving us
reproducibility and avoiding increased rebuilds when the Linux
kernel is updated.

This is tracked upstream as QTBUG-136060

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

@@ -263,6 +263,9 @@ stdenv.mkDerivation rec {
       "-DQT_FEATURE_system_sqlite=ON"
       "-DQT_FEATURE_openssl_linked=ON"
       "-DQT_FEATURE_vulkan=ON"
+      # don't leak OS version into the final output
+      # https://bugreports.qt.io/browse/QTBUG-136060
+      "-DCMAKE_SYSTEM_VERSION="
     ]
     ++ lib.optionals (!stdenv.hostPlatform.isDarwin) [
       "-DQT_FEATURE_sctp=ON"

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

@@ -39,8 +39,11 @@ stdenv.mkDerivation (
       ++ (args.propagatedBuildInputs or [ ]);
 
     cmakeFlags =
-      args.cmakeFlags or [ ]
-      ++ lib.optional stdenv.hostPlatform.isDarwin "-DQT_NO_XCODE_MIN_VERSION_CHECK=ON";
+      # 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"
+      ++ args.cmakeFlags or [ ];
 
     moveToDev = false;