Add workaround for prettier bug

prettier creates a node_modules folder when formatting some files, which
it should not as cache is not enabled. Setting the cache to a file works
around this.

See https://github.com/prettier/prettier/issues/13032

builtinModules/builtinFormatters.nix

@@ -20,7 +20,8 @@ in
     formatters = pkgs:
       let
         nixpkgs-fmt = "${pkgs.nixpkgs-fmt}/bin/nixpkgs-fmt";
-        prettier = "${pkgs.nodePackages.prettier}/bin/prettier --write";
+        prettier = "${pkgs.nodePackages.prettier}/bin/prettier --write"
+          + " --cache-location=.prettiercache";
       in
       {
         "*.nix" = mkDefault nixpkgs-fmt;