New review comment by subnut on void-packages repository

https://github.com/void-linux/void-packages/pull/36362#discussion_r835794441

Comment:
> There should be no external commands run on top level of template.

okay, I've removed them in the latest commit.  
But may I know the reason behind this rule? Also, is it mentioned in the [Manual.md](https://github.com/void-linux/void-packages/blob/master/Manual.md)?