i3status-rust: set 'meta.mainProgram'

pkgs/applications/window-managers/i3/status-rust.nix

···
     description = "Very resource-friendly and feature-rich replacement for i3status";
     homepage = "https://github.com/greshake/i3status-rust";
     license = licenses.gpl3Only;
+    mainProgram = "i3status-rs";
     maintainers = with maintainers; [ backuitist globin ];
     platforms = platforms.linux;
   };