Drop --build-option flag from isolated build script (#2497)

Author: adamchainz

docs/changelog/2497.bugfix.rst

@@ -0,0 +1 @@
+Dropped ``--build-option`` in isolated builds, an alternative fix for the ``SetuptoolsDeprecationWarning`` about using ``--global-option`` -- by :user:`adamchainz`

src/tox/helper/build_isolated.py

@@ -38,5 +38,5 @@ def _ensure_module_in_paths(module, paths):
 if backend_obj:
     backend = getattr(backend, backend_obj)
 
-basename = backend.build_sdist(dist_folder, {"--build-option": ["--formats=gztar"]})
+basename = backend.build_sdist(dist_folder, {})
 print(basename)