Add nixfmt tool, format #33

Job	Run time
style	22s
build (macos-latest, cabal-fmt, nix run .#cabal-fmt -- --help)	9m 57s
build (macos-latest, fourmolu, nix run .#fourmolu -- --help)	1h 11m 11s
build (macos-latest, hlint, nix run .#hlint -- --help)	1h 20m 27s
build (macos-latest, nixfmt, nix run .#nixfmt -- --help)	1m 50s
build (macos-latest, nixpkgs-fmt, nix run .#nixpkgs-fmt)	1m 25s
build (macos-latest, ormolu, nix run .#ormolu -- --help)	57m 6s
build (macos-latest, stylish, nix run .#stylish -- --help)	42m 25s
build (macos-latest, help, nix run .#help)	1m 33s
build (macos-latest, version, nix run .#version)	1m 20s
build (ubuntu-latest, cabal-fmt, nix run .#cabal-fmt -- --help)	5m 21s
build (ubuntu-latest, fourmolu, nix run .#fourmolu -- --help)	33m 5s
build (ubuntu-latest, hlint, nix run .#hlint -- --help)	40m 45s
build (ubuntu-latest, nixfmt, nix run .#nixfmt -- --help)	21s
build (ubuntu-latest, nixpkgs-fmt, nix run .#nixpkgs-fmt)	22s
build (ubuntu-latest, ormolu, nix run .#ormolu -- --help)	30m 30s
build (ubuntu-latest, stylish, nix run .#stylish -- --help)	29m 51s
build (ubuntu-latest, help, nix run .#help)	33s
build (ubuntu-latest, version, nix run .#version)	22s
	6h 48m 46s

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