build: Don't require file existence

Author: ritchie46

py-polars/runtime/_polars_runtime_tmpl/__init__.py

@@ -1,4 +1,7 @@
 # ruff: noqa: TID252
-from .build_feature_flags import BUILD_FEATURE_FLAGS
+try:
+    from .build_feature_flags import BUILD_FEATURE_FLAGS
+except ImportError:
+    BUILD_FEATURE_FLAGS = ""
 
 __all__ = ["BUILD_FEATURE_FLAGS"]