improve build process (wrt release)

The default build process isn't releasing from a fresh export of the repo, this should be changed. It will also have to create package-release.yaml and variant-release.yaml files, and this will take into account per-variant timestamps as well.

Start by tracing the default build process from a fresh export of the repository and identify where release artifacts are generated. Check how package-release.yaml and variant-release.yaml are produced and how per-variant timestamps are handled. Done means a fresh export can complete the release build with both files and the required timestamps.

Written by the indexing model from the issue text.