Fix return setting last exit code (#14120)

# Description

Fixes #14113 and #14112.

# Tests + Formatting

Added a test.

crates/nu-command/tests/commands/return_.rs

@@ -32,3 +32,9 @@ fn return_works_in_script_with_def_main() {
     );
     assert!(actual.err.is_empty());
 }
+
+#[test]
+fn return_does_not_set_last_exit_code() {
+    let actual = nu!("hide-env LAST_EXIT_CODE; do --env { return 42 }; $env.LAST_EXIT_CODE?");
+    assert!(matches!(actual.out.as_str(), ""));
+}