Welcome @nekopep!
Because Pijul is such a young project it is not available in all distributions. It is surprising that Nix on Debian is so hard to install. Nix is an independent package manager (although there’s an entire Linux distribution base on this). If you like it you could provide feedback in there GitHub repo.
Now regarding Pijul itself: yes, you can ignore files by creating an .ignore or .pijulignore file in the root of your project. The syntax is the one used by Git.
I wrote a tutorial for Git users that covers ignoring files. If you have feedback on it I would be super happy to address it.