fix: Ensure all users are read from and that the config is shared between module levels

fleet/default.nix

@@ -2,6 +2,8 @@
   inputs,
   lib,
   config,
+  self ? null,
+  users ? {},
   ...
 }:
 
@@ -139,13 +141,19 @@ let
           typeModule
           overrideModule
           { networking.hostName = hostName; }
+          {
+            # Inject user definitions from flake-parts level
+            config.athenix.users = lib.mapAttrs (_: user: lib.mapAttrs (_: lib.mkDefault) user) users;
+          }
         ]
         ++ lib.optional (externalModulePath != null) externalPathModule;
     in
     {
       system = lib.nixosSystem {
         inherit system;
-        specialArgs = { inherit inputs; };
+        specialArgs = {
+          inputs = if self != null then inputs // { inherit self; } else inputs;
+        };
         modules = allModules;
       };
       modules = allModules;