This pull request refactors the GitHub Actions workflow for packaging the core, splitting the build pipeline into more granular jobs, improving artifact handling and cleanup, and enhancing error reporting in packaging scripts.

The main goals are to improve parallelization and provide clearer diagnostics during packaging:

• The workflow is split into distinct jobs:build-env (fetches the Zephyr environment only once),build-board (builds board-specific binaries in parallel),package-core (packages each core artifact in paraallel), andtest-core (tests each board in parallel). This change nowmakes the whole CI pipeline basically independent of the number of boards / packages being tested.

• CI now tries to run as much as possible in case an error is detected, while still indicating which parts of the CI build are failing.Build failures for individual boards are summarized and included in the GitHub Actions step summary, andthe full contents of the build directory can be retrieved for easy inspection. Tests are attempted even when some of the boards did not compile successfully.

Seehere for a sample run that includes a failed Giga build.

Future work could move the Zephyr env preparation step using Docker; I think that should speed up the process compared to the current artifact upload/download.