ci: github: bump dawidd6/action-download-artifact from 6 to 8

Bumps [dawidd6/action-download-artifact](https://github.com/dawidd6/action-download-artifact) from 6 to 8.
- [Release notes](https://github.com/dawidd6/action-download-artifact/releases)
- [Commits](https://github.com/dawidd6/action-download-artifact/compare/v6...v8

)

---
updated-dependencies:
- dependency-name: dawidd6/action-download-artifact
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>