doc/faq: add nix channel setting

Follow-up to Github issue #175

Author: astro

doc/src/faq.md

@@ -111,3 +111,14 @@ environment.systemPackages = [ (
   ''
 ) ];
 ```
+
+# Can I include my host's `<nixpkgs>` channel when building the VM?
+
+Use the following configuration if you build your MicroVM with
+channels, not Flakes:
+
+```nix
+nix.nixPath = [
+  "nixpkgs=${builtins.storePath <nixpkgs>}"
+];
+```