help: nix-channels: prevent auto appending

Add code to demonstrate how to prevent the automatic appending of the default cache site, which can be especial helpful for users who are completely unable to reach the site:

See also: https://github.com/NixOS/nixpkgs/issues/158356#issuecomment-1556882689

help/_posts/2023-03-10-nix-channels.md

@@ -90,6 +90,21 @@ mirrorid: nix-channels
 </code>
 </pre>
 
+如果因为无法访问 https://cache.nixos.org 等原因，希望避免自动添加该默认地址，请在配置中使用`lib.mkForce`。
+
+{% raw %}
+<script id="template-2-1" type="x-tmpl-markup">
+	  # load `lib` into namespace at the file head with `{ config, pkgs, lib, ... }:`
+    nix.settings.substituters = lib.mkForce [ "{{http_protocol}}{{mirror}}/store" ];
+    </script>
+{% endraw %}
+
+<p></p>
+
+<pre>
+<code id="content-2-1" class="language-nix" data-template="#template-2-1" data-select="#http-select,#sudo-select">
+</code>
+</pre>
 
 #### 临时使用