support NU_USE_IR in benchmarks

benches/benchmarks.rs

@@ -45,6 +45,10 @@ fn setup_stack_and_engine_from_command(command: &str) -> (Stack, EngineState) {
     };
 
     let mut stack = Stack::new();
+
+    // Support running benchmarks with IR mode
+    stack.use_ir = std::env::var_os("NU_USE_IR").is_some();
+
     evaluate_commands(
         &commands,
         &mut engine,