modules: system: users: make it configurable

Notably, make use of my global 'username' option.

modules/system/users.nix

@@ -1,17 +1,27 @@
 # User setup
 { config, lib, pkgs, ... }:
 let
-  my = config.my;
+  secrets = config.my.secrets;
+  cfg = config.my.system.users;
   groupExists = grp: builtins.hasAttr grp config.users.groups;
   groupsIfExist = builtins.filter groupExists;
 in
 {
-  users.mutableUsers = false; # I want it to be declarative.
+  options.my.system.users = with lib; {
+    enable = my.mkDisableOption "user configuration";
+  };
 
-  # Define user accounts and passwords.
+  config = lib.mkIf cfg.enable {
-  users.users.root.hashedPassword = my.secrets.users.root.hashedPassword;
+    users = {
-  users.users.ambroisie = {
+      mutableUsers = false; # I want it to be declarative.
-    hashedPassword = my.secrets.users.ambroisie.hashedPassword;
+
+      users = {
+        root = {
+          inherit (secrets.users.root) hashedPassword;
+        };
+
+        ${config.my.username} = {
+          inherit (secrets.users.${config.my.username}) hashedPassword;
           description = "Bruno BELANYI";
           isNormalUser = true;
           shell = pkgs.zsh;
@@ -33,4 +43,7 @@ in
             in
             keys;
         };
+      };
+    };
+  };
 }