flake: define flake-parts options for colmena

This allows us to define options on the top level, so that we can write
modules to generate config for multiple hosts.

Idea from
1c51514586

flake.nix

@@ -70,7 +70,7 @@
 
   outputs = inputs@{ self, nixpkgs, nixpkgs-small, nixpkgs-unstable, flake-parts, colmena, ... }:
     flake-parts.lib.mkFlake { inherit inputs; } {
-      imports = [ ];
+      imports = [ ./flake ];
 
       systems = [ "x86_64-linux" "aarch64-linux" ];