`nix flake update` to system revision

Sorry to bump this old thread, but the approach described in My painpoints with flakes - #14 by hmenke requires me to use

{
  descripttion = "a dev flake with nixpkgs pinned to system";
  inputs.nixpkgs.url = "nixpkgs";
  outputs =  {};
}

but I prefer inputs.nixpkgs.url = "github:nixos/nixpkgs/<branch>"; as per your follow up comment.

Is there no way initialize flake.lock so it points to the same commit as the system is using and still use the gtihub:nixos/nixpkgs/ syntax?