miniogre update to update pre-existing reproducibility artifacts

[wilderlopes]

Why

It is useful to update the artifacts during the work session, for example, when the developer installs something manually and wants to update the artifacts before committing them.

Features

It differs from miniogre run --no-container because it allows the user to generate each artifact separately. For example:

miniogre update --artifact requirements.txt: updates only the requirements.txt
miniogre update --artifact README.md: updates only the requirements.txt

But the regular miniogre update will update all the existing artifacts.