Clippy fixes for toolchain bump (#13497)

- **Suggested default impl for the new `*Stack`s**
- **Change a hashmap to make clippy happy**
- **Clone from fix**
- **Fix conditional unused in test**
- then **Bump rust toolchain**

crates/nu-protocol/src/engine/argument.rs

@@ -71,7 +71,7 @@ impl Argument {
 }
 
 /// Stores the argument context for calls in IR evaluation.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct ArgumentStack {
     arguments: Vec<Argument>,
 }

crates/nu-protocol/src/engine/engine_state.rs

@@ -428,13 +428,13 @@ impl EngineState {
             .1
     }
 
-    pub fn render_env_vars(&self) -> HashMap<&String, &Value> {
-        let mut result = HashMap::new();
+    pub fn render_env_vars(&self) -> HashMap<&str, &Value> {
+        let mut result: HashMap<&str, &Value> = HashMap::new();
 
         for overlay_name in self.active_overlay_names(&[]) {
             let name = String::from_utf8_lossy(overlay_name);
             if let Some(env_vars) = self.env_vars.get(name.as_ref()) {
-                result.extend(env_vars);
+                result.extend(env_vars.iter().map(|(k, v)| (k.as_str(), v)));
             }
         }
 

crates/nu-protocol/src/engine/error_handler.rs

@@ -10,7 +10,7 @@ pub struct ErrorHandler {
 }
 
 /// Keeps track of error handlers pushed during evaluation of an IR block.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct ErrorHandlerStack {
     handlers: Vec<ErrorHandler>,
 }