modules: system: users: use 'ambroisie' password

Do not rely on `my.user.name` which could be changed to a value not
available in the secrets.

modules/system/users.nix

@@ -21,7 +21,7 @@ in
         };
 
         ${config.my.user.name} = {
-          inherit (secrets.users.${config.my.user.name}) hashedPassword;
+          inherit (secrets.users.ambroisie) hashedPassword;
           description = "Bruno BELANYI";
           isNormalUser = true;
           shell = pkgs.zsh;