Make get_env_var return a reference to a Value (#13987)

# Description
Title says it all, changes `EngineState::get_env_var` to return a
`Option<&'a Value>` instead of an owned `Option<Value>`. This avoids
some unnecessary clones.

I also made a similar change to the `PluginExecutionContext` trait.

src/test_bins.rs

@@ -348,7 +348,7 @@ pub fn nu_repl() {
                 .coerce_str()
                 .unwrap_or_else(|err| outcome_err(&engine_state, &err));
             let _ = std::env::set_current_dir(path.as_ref());
-            engine_state.add_env_var("PWD".into(), cwd);
+            engine_state.add_env_var("PWD".into(), cwd.clone());
         }
         top_stack = Arc::new(Stack::with_changes_from_child(top_stack, stack));
     }