chore(nix): better formatting job (#1282)

one that doesn't install a bunch of unrelated stuff just to format

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

.github/workflows/nix.yml

@@ -28,4 +28,4 @@ jobs:
         uses: cachix/install-nix-action@v26
         with:
           nix_path: nixpkgs=channel:nixos-unstable
-      - run: nix develop -c make nix-fmt
+      - run: nix develop .#fmt -c make nix-fmt

flake.nix

@@ -149,6 +149,13 @@
 
           release =
             pkgs_4_14.mkShell { buildInputs = [ pkgs_4_14.dune-release ]; };
+
+          fmt = let pkgs = (import nixpkgs { inherit system; });
+          in pkgs.mkShell {
+            # TODO: get rid of ocaml once dune get format without ocaml being
+            # present
+            buildInputs = (with pkgs; [ ocaml ocamlformat_0_26_1 yarn dune_3 ]);
+          };
         };
       }));
 }