pigz: add meta.mainProgram

pkgs/by-name/pi/pigz/package.nix

···
   meta = with lib; {
     homepage = "https://www.zlib.net/pigz/";
     description = "Parallel implementation of gzip for multi-core machines";
+    mainProgram = "pigz";
     maintainers = [ ];
     license = licenses.zlib;
     platforms = platforms.unix;