gnugrep: add meta.mainProgram

Fix the following error when running `nix run .#gnugrep`:
error: unable to execute '/nix/store/p21zz3jvnsk6h7pb83lm9hyl2hb7d2g8-gnugrep-3.6/bin/gnugrep': No such file or directory

pkgs/tools/text/gnugrep/default.nix

@@ -60,6 +60,7 @@ stdenv.mkDerivation {
 
     maintainers = [ maintainers.eelco ];
     platforms = platforms.all;
+    mainProgram = "grep";
   };
 
   passthru = {inherit pcre;};