Avoids `UnboundLocalError`:
```
cannot access local variable 'sysroot' where it is not associated with a value
```

Also fixes a latent call-site bug where `download_bazel_task_artifacts` was being called with two extra positional arguments (`multiversion_dir` and `self.execution`) that the function does not accept. These were silently binding to the `retry_secs` and `download_timeout_secs` parameters respectively, causing `download_timeout_secs` to be set to the task execution number (typically 0 or 1) instead of the intended 30-minute default.