use exec as default mode

crates/nit/src/config.rs

@@ -34,7 +34,7 @@ pub fn get_config() -> Result<Config> {
             m => panic!("Bad mode: {m}"),
         }
     } else {
-        Mode::Spawn
+        Mode::Exec
     };
 
     let platform = platform::get_current_platform(values.remove("platform").as_deref())?;