rustbuild: Rustbuild tests spuriously fail in ~50% cases on Windows

With errors like this:
```
thread 'builder::__test::dist_with_targets_and_hosts' panicked at 'fs::create_dir_all(&dst) failed with Access is denied. (os error 5)', bootstrap\dist.rs:103:9
```

Perhaps rustbuild tests have some filesystem races with each other, or perhaps they need to wait a bit and retry if error happens during directory creation.

cc @Mark-Simulacrum 