docs: update `actions/download-artifact` reference

As of late last year, `actions/download-artifact` released version 4 which now supports immediate downloading after uploading.

https://github.blog/changelog/2023-12-14-github-actions-artifacts-v4-is-now-generally-available/
https://github.com/actions/download-artifact/compare/v3...v4.0.0#diff-5a373a8bb1123125c60fb78083d18f37b02afed7c72ce73c441ce283f66bb80c