Add fourmolu to the nix devshell

Use that in the CI

Author: andreabedini

.github/workflows/formatting.yaml renamed to .github/workflows/format.yaml

@@ -10,7 +10,5 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-
-      - uses: haskell-actions/run-fourmolu@v9
-        with:
-          version: "0.14.0.0"
+      - uses: cachix/install-nix-action@v22
+      - run: nix develop --command fourmolu -m check .

flake.nix

@@ -37,6 +37,7 @@
             cabal = "latest";
             hlint = "latest";
             haskell-language-server = "latest";
+            fourmolu = "0.14.0.0";
           };
         };