Update failing test due to latest versions of pip_run

It seems that the latest version of `pip_run` can no longer handle `str`
objects in `inject_sitecustomize` (see
`https://github.com/jaraco/pip-run/commit/30c62a0eadd5af422709e75993ff19a39fe69733`).

Author: abravalheri

setuptools/tests/test_editable_install.py

@@ -341,7 +341,7 @@ def test_editable_with_prefix(tmp_path, sample_project, editable_opts):
     site_packages.mkdir(parents=True)
 
     # install workaround
-    pip_run.launch.inject_sitecustomize(str(site_packages))
+    pip_run.launch.inject_sitecustomize(site_packages)
 
     env = dict(os.environ, PYTHONPATH=str(site_packages))
     cmd = [