added missing reference

flake.nix

@@ -80,7 +80,7 @@ rust-project TODO: write shell script for automatically updating `cargoHash`
 
           postFixup = ''
             wrapProgram $out/bin/time_tracker \
-              --prefix PATH : "${lib.makeBinPath [ msmtp ]}"
+              --prefix PATH : "${nixpkgs.lib.makeBinPath [ msmtp ]}"
           '';
         });
       };