This gives us Nix 2.30 as `nixVersions.latest`, which enables it for
Eval in CI automatically.
It also gives us markdown-code-runner 0.2.0, which allows it to run with
treefmt.
From the nixpkgs-unstable channel:
https://hydra.nixos.org/eval/1817362#tabs-inputs
Changes for treefmt-nix:
421b56313c...58bd4da459
(cherry picked from commit 492225f4f7)
Instead of rolling our own update script which only works for a single
pin, let's use npins. We can then use it for the treefmtNix pin as well,
which was mostly unmaintained, so far.
(cherry picked from commit f7c25f0bf9)
