Hmmh, still not working. I have the following relevant options set, what could be missing?

nix = {
  extraOptions = ''
    allowed-uris = https:// ssh://
  '';
                                                     
  distributedBuilds = true;
  buildMachines = [{
    hostName = "localhost";
    systems = [
      "aarch64-linux"
      "i686-linux"
      "wasm32-wasi"
      "x86_64-linux"
      "x86_64-windows"
    ];
  }];
};
                                                     
boot.binfmt.emulatedSystems =
  [ "aarch64-linux" "wasm32-wasi" "x86_64-windows" ];

we use something along the lines of

    boot.binfmt.registrations = {
      qemu-aarch64 = {
        magicOrExtension = "\\x7fELF\\x02\\x01\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\xb7";
        mask = "\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\x00\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff";
        interpreter = "${pkgs.qemu}/bin/qemu-aarch64";
      };
    };

    nix.sandboxPaths  [ "/run/binfmt" "${pkgs.qemu}" ];