Make build script compatible with Python < 3.9.

PiperOrigin-RevId: 519206113

pip_build.py

@@ -442,7 +442,7 @@ def test_wheel(wheel_path, expected_version, requirements_path):
     parser = argparse.ArgumentParser()
     parser.add_argument(
         "--nightly",
-        action=argparse.BooleanOptionalAction,
+        action="store_true",
         help="Whether this is for the `keras-nightly` package.",
     )
     args = parser.parse_args()