Update Nix configuration

[Niols]

• Update Nix flake dependencies and ignore unused inputs.
• Use newly added fixed dune-fmt hook (fixed by Wrap dune fmt in a clean written tool cachix/git-hooks.nix#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.