Benchmark baselines aren't being saved

Running the benchmarks with this command should save the results for comparison against other runs, but I can't get that to work:

```
$ xcrun --toolchain org.swift.59202403071a swift package -c release --allow-writing-to-package-directory benchmark baseline update main
```

I see the progress output of the benchmark run, but nothing is saved anywhere that I can see.

Run the provided xcrun benchmark baseline update command and confirm that benchmark progress appears without a saved result. Trace the baseline update path to determine where comparison data should be written; done means the run saves a baseline that can be found and used for comparison against later runs.

Written by the indexing model from the issue text.