[native_assets_builder] Purge old directories

The native assets builder creates directories in the form of `.dart_tool/native_assets_builder/`. These directories should be purged at some point:

https://github.com/dart-lang/native/blob/4c5fca965acecc0256489c9b0398518629cb9045/pkgs/native_assets_builder/lib/src/build_runner/build_runner.dart#L255

Possible purging strategy:

* `touch` the `config.json` (or a `last_read.json`) every time it's read. And with a 1% chance delete everything that hasn't been read in a week.