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 .
         '';
       };
+  };
 }