Don't use isolated builds when building an sdist for a pipeline packa…

…ge supplied in --setup_file option. (#31070)

sdks/python/apache_beam/runners/portability/stager.py

@@ -791,6 +791,7 @@ def _build_setup_package(setup_file,  # type: str
               Stager._get_python_executable(),
               '-m',
               'build',
+              '--no-isolation',  # Otherwise, we need internet access to PyPI.
               '--sdist',
               '--outdir',
               temp_dir,