pigz: add meta.mainProgram

Changed files
+1
pkgs
by-name
pi
+1
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;