fix "Sphinx 8 will drop support for representing paths as strings"

/home/runner/work/sssd.io/sssd.io/extensions/sssd/io/postprocess/search.py:58: RemovedInSphinx80Warning: Sphinx 8 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.
  relpath = path[len(build_dir) + 1:]

extensions/sssd/io/postprocess/search.py

@@ -55,7 +55,7 @@ def build_search_index(app, exception):
 
     docs = []
     for path in glob.glob(f'{build_dir}/**/*.html', recursive=True):
-        relpath = path[len(build_dir) + 1:]
+        relpath = path[len(str(build_dir)) + 1:]
         if relpath in exclude_pages:
             continue