Try to use a shell.nix

See: https://ghedam.at/a-tour-of-nix-flakes

flake.nix

@@ -7,13 +7,7 @@
       flake-utils.lib.eachDefaultSystem (system:
       let pkgs = nixpkgs.legacyPackages.${system}; in
       rec {
-        devShell = pkgs.mkShell {
-          buildInputs = with pkgs; [ lessc minify ];
-
-          shellHook = ''
-            echo "shell with lessc and minify"
-          '';
-        };
+        devShell = import ./shell.nix {inherit pkgs; };
       }
     );
 }

shell.nix

@@ -0,0 +1,11 @@
+{ pkgs ? import <nixpkgs> {} }:
+with pkgs;
+mkShell {
+  buildInputs = [ 
+    lessc 
+    minify 
+  ];
+  shellHook = '' 
+    echo "shell with lessc and minify"
+    '';
+}