this removes a difference to how Nix currently fetches flake references.
using a constant is preferrable because renaming the source won't force
downstream rebuilds of store objects cached from when flakes were used.
upstream fix: https://github.com/nmattia/niv/pull/379
use niv for dependency management.
this follows our own tutorials on writing packages and development
shells, and we generally find it is easier to read and work with.
Co-authored-by: Alejandro Sanchez Medina <alejandrosanchzmedina@gmail.com>