put shellHook in mkShell call

flake.nix

@@ -10,9 +10,7 @@
   {
     devShells."${system}" =
     let
-      pkgs = import nixpkgs {
+      pkgs = nixpkgs.legacyPackages."${system}";
-        inherit system;
-      };
     in
       pkgs.mkShellNoCC {
         packages = with pkgs; [
@@ -31,10 +29,10 @@
             ];
           })
         ];
+        shellHook = ''
+          echo "starting codium"
+          codium .
+        '';
       };
-      shellHook = ''
-        echo "starting codium"
-        codium .
-      '';
   };
 }