Maybe I'm misunderstanding it but I think the manual is simply 
mistaken:

   “Unless you're using Guix System, the 'guix install' command 
   must have
    printed this hint:”

amuza 写道：
>  hint: Consider setting the necessary environment variables by 
>  running:

I expect this to happen on Guix System, for the same reason it 
happens on other distributions: Guix can't sanely modify the 
environment of its parent shell or other already running 
processes.

Kind regards,

T G-R