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
... and remove a few unused arguments.
This adds pkg-config to a two or three packages that don't use it,
but we shouldn't let that bother us. It's like our personal stdenv.
They are not actually part of the store layer, but instead part of the
Nix executable infra (libraries don't need plugins, executables do).
This is part of a larger project of moving all of our legacy settings
infra to libmain, and having the underlying libraries just have plain
configuration structs detached from any settings infra / UI layer.
Progress on #5638
