Follow ARTIQ, and in this project lets us include the version number
directly in flake.nix instead of linking to the toml file of a specific
release date, as we use stable Rust.
Also, from nixpkgs manual:
both oxalica's overlay and fenix better integrate with nix and cache
optimizations. Because of this and ergonomics, either of those
community projects should be preferred to the Mozilla's Rust overlay
(nixpkgs-mozilla).
