Bump fourmolu to latest version

[ngua]

After #300 is merged, we'll be using Nix to install fourmolu for the formatting workflow. The version on nixpkgs stable is 0.3, but we had been using 0.4 via stack. Unfortunately, nixos/nixpkgs/nixpkgs-unstable#haskellPackages.fourmolu_0_4_0_0 is marked as broken due to the version ranges of its Cabal and ghc-lib-parser dependencies.

We can try to override the version bounds with doJailbreak, etc... or try to solve the underlying issue and perhaps contribute back upstream