added nix-shell compatibility

flake.nix

@@ -80,10 +80,6 @@
       ];
     };
 
-    devShells.${system}.default = with pkgs; mkShell {
+    devShells.${system}.default = pkgs.callPackage ./shell.nix {};
-      packages = [
-        just
-      ];
-    };
   };
 }

shell.nix

@@ -0,0 +1,8 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+with pkgs;
+mkShell {
+  packages = [
+    just
+  ];
+}