use nixfmt-tree instead of calling the nixfmt-rfc-style directly (#2928)

as of nix 2.25.0, `nix fmt` will not automatically recurse through
nix files so `nixfmt-rfc-style` will format from stdin instead.

see: https://github.com/NixOS/nix/pull/11438

this issue https://github.com/NixOS/nixfmt/issues/273 recommends that
we use `nixfmt-tree` which calls `treefmt` in the backend with a
minimal configuration for nix

flake.nix

@@ -248,6 +248,6 @@
           isLLDB = true;
         }
       );
-      formatter = forAllSystems (system: pkgsBySystem.${system}.nixfmt-rfc-style);
+      formatter = forAllSystems (system: pkgsBySystem.${system}.nixfmt-tree);
     };
 }