qtwebengine does not build reproducibly

When building qtwebengine twice, the resulting `lib/libQt6WebEngineCore.so.6.9.0` is not bit-by-bit identical.

diff <(nm /nix/store/n0jw07kni8871kxafv9gkzazvjdz3yvi-qtwebengine-6.9.0/lib/libQt6WebEngineCore.so.6.9.0 | cut -d " " -f 2) <(nm /nix/store/n0jw07kni8871kxafv9gkzazvjdz3yvi-qtwebengine-6.9.0.check/lib/libQt6WebEngineCore.so.6.9.0 | cut -d " " -f 2)

suggests the same symbols are there in the same order, but at different addresses.

I'm pretty sure this is not a distro-specific issue, but I encountered it in NixOS, the issue in the distro-specific tracker is https://github.com/NixOS/nixpkgs/issues/393832