How to cross-compile with a custom stdenv on Nix

nix_cross_custom_stdenv.nix

{
  outputs = { nixpkgs, ... }: {
    pkgsWeb = import nixpkgs {
      system = "x86_64-linux";
      # NOTE: enabling setting crossSystem disables config.replaceStdenv
      crossSystem = {
        config = "wasm32-unknown-none";
        rust.rustcTarget = "wasm32-unknown-unknown";
        useLLVM = true;
      };
      # see: nixpkgs/pkgs/stdenv/cross/default.nix
      config.replaceCrossStdenv = { buildPackages, baseStdenv }: baseStdenv // { cc = buildPackages.llvmPackages_19.clang; };
      config.allowUnsupportedSystem = true;
    };
  };
}