Use some kind of synchronization primitive when copying files for prebuilt detection

https://github.com/dotnet/installer/blob/4106448ffae0ac01c5cdec31608b7904aaf1ef05/src/SourceBuild/content/repo-projects/Directory.Build.targets#L495-L511 copies restored packages from the inner build to a global cache. Because many builds restore the same packages, it's very possible to race and cause locking issues.

Retries aren't a great solution here because they cause warnings, which can then get picked up as errors. Instead, if a global shared folder is desirable, a custom copy task should be used that handles synchronization between parallel build processes.