hash guix after guix pull

[Hamzeh Nasajpour]

Hi,

I want to use a custom application for running `guix pull` and `reconfigure` 
after each OS start up.
I developed a python library and put it in the configuration to run it as a 
single-shot service as root.
It will be run but seems that I should update `PATH` env via `hash guix` since 
some of my packages in my custom channel won't be loaded.

```
...
no code for module (px package ...)
--

If I run my python library manually everything works fine (guix pull & 
reconfigure) but I can't run it as a service.

Do you have any idea? Is there anything in somewhere that I missed?

Regards,
Hamzeh Nasajpour
PantherX Team