flake: add overridable systems input

Users can specify which systems to use by having our `systems` input
follow a different file or flake:

```nix
{
  inputs = {
    # Here we list systems in a local file
    inputs.systems.url = "path:./systems.nix";
    inputs.systems.flake = false;

    inputs.nixvim.url = "github:nix-community/nixvim";
    # Here we override the list of systems with only our own
    inputs.nixvim.inputs.systems.follows = "systems";

    # ...
  };

  outputs = inputs: {
    # ...
  };
}
```

Alternatively, instead of users listing systems in a local file, they
can use an external flake, e.g.:

- github:nix-systems/default
  - Exposes aarch64 and x86_64 for linux and darwin
- github:nix-systems/default-linux
  - Exposes aarch64 and x86_64 for linux
- github:nix-systems/default-darwin
  - Exposes aarch64 and x86_64 for darwin
- github:nix-systems/aarch64-darwin
- github:nix-systems/aarch64-linux
- github:nix-systems/x86_64-darwin
- github:nix-systems/x86_64-linux

See https://github.com/nix-systems/nix-systems

flake.nix

@@ -4,6 +4,8 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
 
+    systems.url = "github:nix-systems/default";
+
     flake-parts = {
       url = "github:hercules-ci/flake-parts";
       inputs.nixpkgs-lib.follows = "nixpkgs";
@@ -26,13 +28,7 @@
   outputs =
     inputs:
     inputs.flake-parts.lib.mkFlake { inherit inputs; } {
-      systems = [
-        "x86_64-linux"
-        "aarch64-linux"
-        "x86_64-darwin"
-        "aarch64-darwin"
-      ];
-
+      systems = import inputs.systems;
       imports = [ ./flake ];
     };
 }