Nix makes it easier to have a reproducible development environment on any Linux distribution.
It doesn't interfere with other installed tooling, and is just an additional, optional way to build flexisip.
I have pinned a version of nixpkgs as fallback in case new package versions would break things, and have verified it builds with a --pure shell and clean build folder.
The files have been formatted with nixpkgs-fmt (which is included in the default shell) and should be kept that way for consistency.