fix: re-arange arguments to constructSystem

this lead to https://github.com/tgirlcloud/easy-hosts/issues/3

flake-module.nix

···
 
                 system = mkOption {
                   type = types.str;
-                  default = constructSystem cfg config.class config.arch;
+                  default = constructSystem cfg config.arch config.class;
                   example = "aarch64-darwin";
                   description = "The system to be used for the host";
                   internal = true; # this should ideally be set by easy-hosts

lib.nix

···
     cfg: class: ({ linux = "nixos"; } // (cfg.additionalClasses or { })).${class} or class;
 
   constructSystem =
-    config: class: arch:
+    config: arch: class:
     let
       class' = redefineClass config class;
       os = classToOS class';