Add dev shell with nixfmt

flake.nix

@@ -75,5 +75,15 @@
         modules = [ ./hosts/test-vm/configuration.nix ];
       };
     };
+
+    devShells = let
+      system = "x86_64-linux";
+      pkgs = inputs.nixpkgs.legacyPackages.${system};
+    in
+      {
+        ${system}.default = pkgs.mkShell {
+          packages = [ pkgs.nixfmt-rfc-style ];
+        };
+      };
   };
 }