[8.0.0] Properly cancel repository downloads

[bazel-io]

Forked from #23837

[moroten]

I hope this will make it to 8.0.0 so that Windows users can use default values for --experimental_worker_for_repo_fetching and --enable_bzlmod without randomly getting "directory not empty" and that Linux users get better cache hit ratio when there is no longer rubbish from previously not properly cancelled downloads.