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

[mmitche]

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.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mthalman]

Rather than doing any fancy synchronization it might be easiest for each repo to output to it's own staging directory which are then consolidated after all repos are done building.

[mthalman]

Rather than doing any fancy synchronization it might be easiest for each repo to output to it's own staging directory which are then consolidated after all repos are done building.

I guess this wouldn't work if using the clean-while-building option since the files would get deleted before it got to the end of the build.