Fix clippy warnings

Author: dhruvmanila

crates/ruff/src/checkers/ast/analyze/deferred_scopes.rs

@@ -216,7 +216,7 @@ pub(crate) fn deferred_scopes(checker: &mut Checker) {
             }
         }
 
-        if checker.is_stub {
+        if checker.source_type.is_stub() {
             if checker.enabled(Rule::UnusedPrivateTypeVar) {
                 flake8_pyi::rules::unused_private_type_var(checker, scope, &mut diagnostics);
             }