> as I sayd above, I still don't know how to skip loading packages from
> ~/.emacs.d/elpa when in "guix emacs"

To test if Emacs is from Guix, you can

--8<---------------cut here---------------start------------->8---
  (if (require 'guix-emacs nil 'noerror)
	    ; Code for Guix Emacs
		; Code for non-Guix Emacs
		)
--8<---------------cut here---------------end--------------->8---

In particular, you can set `package-user-dir' and call (package-initialize) for
non-Guix Emacs.

> when using the "native" one, emacs could be started using a different
> user-emacs-directory: this way emacs installed packages (from
> ELPA/MELPA or manually) will be available only in that environment
> 
> but this solution is not good for me, since I should maintain two
> different config trees: one for each emacs... no no no :-O

Indeed, just set different `package-user-dir' and keep the same config :)

-- 
Pierre Neidhardt
https://ambrevar.xyz/