Update workflow cpp-serde-compat-yml

[leerho]

This Java GHA CI workflow (in brief):

• Checks out C++, builds it and runs all the tests
• Some of those tests generate sketch image files of the form ‘*.sk’.
• Copies just those *.sk files into a directory ‘serialization_test_data/cpp_generated_files’ where Java can read them.
• Sets up Java to run tests using the java profile ‘check-cpp-files’

This all works ok, but I have modified this workflow to provide a downloadable Artifact at the end whereby the user can download the directory that contains all the CPP generated sketch images. It is very useful to have an up-to-date copy of these CPP generated files for local testing purposes. The Artifact will be deleted after 30 days.