rustPlatform.fetchCargoVendor: decrease concurrency

    out_dir.mkdir(exist_ok=True)

    shutil.copy(lockfile_path, out_dir / "Cargo.lock")

    # create a pool with at most 10 concurrent jobs

    with mp.Pool(min(10, mp.cpu_count())) as pool:

    # fetch git trees sequentially, since fetching concurrently leads to flaky behaviour

    if len(git_packages) != 0:

        (out_dir / "git").mkdir()

            # run download jobs in parallel

            git_args_gen = ((url, git_sha_rev, out_dir) for git_sha_rev, url in git_sha_rev_to_url.items())

            pool.starmap(download_git_tree, git_args_gen)

        for git_sha_rev, url in git_sha_rev_to_url.items():

            download_git_tree(url, git_sha_rev, out_dir)

    # run tarball download jobs in parallel, with at most 5 concurrent download jobs

    with mp.Pool(min(5, mp.cpu_count())) as pool:

        if len(registry_packages) != 0:

            (out_dir / "tarballs").mkdir()

            # run download jobs in parallel

            tarball_args_gen = ((pkg, out_dir) for pkg in registry_packages)

            pool.starmap(download_tarball, tarball_args_gen)