Follow nix/nixpkgs

Dependencies

• [2] J52POHVC Update to NixOS 20.09
• [3] RWNXH3H2 lastModified -> lastModifiedDate

Change contents

• replacement in flake.nix at line 4
  [3.81]→[2.202:248](∅→∅)
  − inputs.nixpkgs.url = "nixpkgs/nixos-20.09";
  [3.81]

  [3.127]
  + inputs.nixpkgs.follows = "nix/nixpkgs";