Support version being shared between fetcher revision and derivation

[expipiplus1]

like https://github.com/nixos/nixpkgs/blob/master/pkgs/tools/misc/tmux/default.nix#L24-L33

stdenv.mkDerivation rec {
  pname = "tmux";
  version = "3.1b";

  outputs = [ "out" "man" ];

  src = fetchFromGitHub {
    owner = "tmux";
    repo = "tmux";
    rev = version;
    sha256 = "0jvyq4r691bn0wsr8i6c0q0lzss25vm9nx8sv3fhw9cs63ncq04y";
  };

This should update the version expression and not the rev one.

[expipiplus1]

actually in general if anything to be updated is a variable then that variable should be updated.