Multi-System with flakes

I think it makes sense to preserve this _beautiful_ simplicity:

```nix
    (makes.lib.flakes.evaluate { inherit inputs; system = "x86_64-linux"; })
```

if more system are to be supported, I think `makes` should follow it's own `system` namespacing scheme, implemented behind:

```nix
    (makes.lib.flakes.evaluate { inherit inputs; systems = supportedSystems; })
```