Update Nix configuration

[Niols]

• Update Nix flake dependencies and ignore unused inputs.
• Use newly added fixed dune-fmt hook (fixed by https://github.com/cachix/pre-commit-hooks.nix/pull/296).
• Have one devShell per package, for people who'd want their development environment handled by opam-nix as well.
• Add descriptions to each package in the flake, such that nix search . returns slightly useful information.