Then, I ran `./pre-inst-env guix shell emacs-next tree-sitter-c 
tree-sitter-cpp --container --preserve="^TERM$" -- emacs -Q -nw` 
and executed the following code inside that Emacs session:

--8<---------------cut here---------------start------------->8---
(defvar guix-treesit-grammar-paths
  (let ((grammars-path (if (getenv "TREE_SITTER_GRAMMARS")
                           (getenv "TREE_SITTER_GRAMMARS")
                         "")))
    (split-string grammars-path ":")))

(if (treesit-available-p)
    (setq treesit-extra-load-path guix-treesit-grammar-paths))
--8<---------------cut here---------------end--------------->8---

Doing these steps, Emacs' treesit was able to use the grammars 
installed by Guix in c-ts-mode. I don't know if a native 
search-path is the appropriate approach or how to include the 
above elisp (or something similar) in the emacs-next package, but 
I hope this helps Pierre or anyone else with more understanding to 
do the v8 of this patch series :).

Regards,
-- 
Luis H. Higino