added non-flake nix compatibility with flake-compat

[sunworms]

This PR aims to add compatibility for non-flake setups by adding the input NixOS/flake-compat and creating a default.nix and a shell.nix that provide the same outputs as the ones provided by the flake.

[marcusramberg]

looks ok.

[LuckShiba]

The addition looks okay, but I'm thinking about how necessary that is. Users that doesn't want to use the flake should in a best-case scenario use the Nixpkgs module, but it's still not in stable so I guess that is okay.

The only problem here is that it'll add two new top-level Nix files, which is okay but not ideal. @sunworms, I don't know much about non-flake usage, would it be viable having these files inside distro/nix or not at all?

[LuckShiba]

By testing here it seems to work by moving the files to distro/nix, changing the paths to reference ../../ and running nix-shell distro/nix or nix-build distro/nix, couldn't that be better?

[Purian23]

@sunworms have you looked into the last comment by LuckShiba? I would like to get this in or cleared out if we can.