rush: add meta.mainProgram

Suppress warnings when using `lib.getExe`.

Author: c4f3z1n

pkgs/by-name/ru/rush/package.nix

@@ -26,6 +26,7 @@ stdenv.mkDerivation rec {
   doCheck = true;
 
   meta = {
+    mainProgram = "rush";
     broken = stdenv.hostPlatform.isDarwin;
     description = "Restricted User Shell";