Create directories for output JSON files

[bernhardmgruber]

I have ran into this twice now and thought it would be great if an nvbench-based benchmark could create any intermediate directories for the output JSON file.

Now, with the directory latest_json not existing, the benchmark aborts with an error:

./bin/cub.bench.transform.babelstream2.base -d 0 --stopping-criterion entropy --json latest_json/babelstream2.json

NVBench encountered an error:

[...]/cccl/build_profile/_deps/nvbench-src/nvbench/option_parser.cu:587: Error while adding json output for `latest_json/babelstream3.json`:
basic_ios::clear: iostream error