Flake-utils fails to find system attribute

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1

I have the following flake.nix

{
  description = "";

  inputs = {
    nixpkgs.url = "nixpkgs/nixpkgs-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};

        buildPackages = [
          pkgs.cowsay
        ];
      in with pkgs;
      {
        devShells.default =
          mkShell {
            buildInputs = buildPackages;
          };

      packages.default =
        stdenv.mkDerivation {
          name = "build";
          src = ./.;
          buildInputs = buildPackages;
      };
    }
  );
}

with nix develop I get the following error:

error: flake 'git+file:///.....' does not provide attribute 'devShells.x86_64-linux.devShell.x86_64-linux', 'packages.x86_64-linux.devShell.x86_64-linux', 'legacyPackages.x86_64-linux.devShell.x86_64-linux', 'devShell.x86_64-linux' or 'defaultPackage.x86_64-linux'

What am I doing wrong?

Edit: Also the example from flake-utils seems to fail. Opened an issue in their bugtracker: eachDefaultSystem leads to missing architecture attributes · Issue #114 · numtide/flake-utils · GitHub