This patch updates the flake to track the nixos-unstable branch instead
of master for the nixpkgs flake input.
This ensures that hydra.nixos.org (the NixOS CI) has built all packages,
made them available in the cache.nixos.org binary cache without the risk
of a definitions (evalulation) being broken on master.
Add a nix flake to `Toolchain/` that wraps the existing nix derivation
`Toolchain/serenity.nix`. This also comes with a lockfile making the nix
developer enviornment setup more reproducible.
We also update the documentation for "other builds" to mention the
flake.