Remove engine_state clones in REPL eval (#7713)

A small but easy optimization for `evaluate_repl()`: clone
`engine_state` 1x instead of 3x.

This reduces time spent in a simple REPL eval (`enter` key pressed with
no command text) by about 10%, as measured in
[Superluminal](https://superluminal.eu/).

crates/nu-cli/src/validation.rs

@@ -1,9 +1,10 @@
 use nu_parser::{parse, ParseError};
 use nu_protocol::engine::{EngineState, StateWorkingSet};
 use reedline::{ValidationResult, Validator};
+use std::sync::Arc;
 
 pub struct NuValidator {
-    pub engine_state: EngineState,
+    pub engine_state: Arc<EngineState>,
 }
 
 impl Validator for NuValidator {