Fix pip seeder with --extra-search-dir specified

pip needs to have `--find-links` CLI argument for each directory
not just once for a list of directories.

Fixes: pypa#1834

src/virtualenv/seed/embed/pip_invoke.py

@@ -43,7 +43,7 @@ def get_pip_install_cmd(self, exe, version):
             folders = set()
             for context in (ensure_file_on_disk(get_bundled_wheel(p, version), self.app_data) for p in pkg_versions):
                 folders.add(stack.enter_context(context).parent)
+            folders.update(set(self.extra_search_dir))
             for folder in folders:
                 cmd.extend(["--find-links", str(folder)])
-                cmd.extend(self.extra_search_dir)
             yield cmd