silver-searcher: add meta.mainProgram

pkgs/tools/text/silver-searcher/default.nix

@@ -22,6 +22,7 @@ stdenv.mkDerivation rec {
     homepage = "https://github.com/ggreer/the_silver_searcher/";
     description = "A code-searching tool similar to ack, but faster";
     maintainers = with maintainers; [ madjar ];
+    mainProgram = "ag";
     platforms = platforms.all;
     license = licenses.asl20;
   };