pngquant: set meta.mainProgram

pkgs/tools/graphics/pngquant/default.nix

···
     changelog = "https://github.com/kornelski/pngquant/raw/${version}/CHANGELOG";
     platforms = platforms.unix;
     license = with licenses; [ gpl3Plus hpnd bsd2 ];
+    mainProgram = "pngquant";
     maintainers = [ maintainers.srapenne ];
   };
 }