executor: fix race condition with forked git repos

When installing packages from different directories of a forked
monorepo, a race condition may occur, where multiple git clients
are interacting in parallel with the same git repository.

Fix by serializing git operations that interact with the same
git repository.

This makes the test succeed. Remove xfail.

Links:
https://github.com/python-poetry/poetry/pull/9658#discussion_r1745925411
https://github.com/orgs/python-poetry/discussions/9718#discussioncomment-10785589