fix: allow missing `source.archive` keyword

This prevents having to do a breaking change of the project.

Previously, the usage was that the integrity of the `git archive`
output was used to match up the dynamically generated source archive
that GitLab (or GitHub) produces.

If we _require_, `archive` then those flows will break.

However, we can easily still support them by writing out to a
placeholder `src.tar` archive.

The documentation has been updated to push people towards providing an
`archive` keyword.