json2nix: use nixfmt internally

This helps with creating better error messages,
because we can now see where in the pipeline
the error occurred

internals/json2nix/haskell-overlay.nix

@@ -0,0 +1,10 @@
+{ pkgs, compiler, ... }:
+final: prev: {
+  haskellPackages = with prev.haskell.lib;
+    prev.haskell.packages.${compiler}.override {
+      overrides = hpFinal: hpPrev: {
+        # Upgrade nixfmts "text" dependency to >= 2
+        nixfmt = doJailbreak hpPrev.nixfmt;
+      };
+    };
+}