[native_assets_builder] Write logs of build.dart invocations

[dcharkes]

When invoking build.dart we stream the output to the logger provided by the Dart/Flutter SDK. This logger is likely not in verbose mode, so only errors will be shown. It might be useful to just always write the full stderr and full stdout logs to the build folder so that they can be easily accessed.

[dcharkes]

Cross referencing logging bugs from Dart Dev and Flutter Tools:

• [native assets] Should surface stdout/stderr from build.dart sdk#53732
• "native-assets" feature should surface stdout/stderr from build.dart flutter/flutter#136386

Always writing logs to the output folder will not solve the above issues by themselves, because it will be unclear where the logs are. The logs will likely somewhere similar to .dart_tool/native_assets_builder/<some_hash_of_the_build_config>/stdout.txt.