perSystem is too eager by default

[figsoda]

# flake.nix
{
  outputs = inputs@{ flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit inputs; } {
      flake.devShells = throw "devShells";

      systems = [
        "aarch64-darwin"
        "aarch64-linux"
        "x86_64-darwin"
        "x86_64-linux"
      ];

      perSystem = { self', ... }: {
        checks = {
          foo.type = "derivation";
          bar = self'.checks.foo;
        };
      };
    };
}

$ nix eval .#checks 
error: devShells
(use '--show-trace' to show detailed location information)