This does not include any automation for the release branch, but
is based on the configuration of https://github.com/NixOS/nix/pull/12349
pre-commit run -a nixfmt-rfc-style
It had gotten rather big. Hopefully we'll eventually have some generic
infra for a "multi-package dev shell" and not need so much code for
this, but until then it's better in a separate file.