bug#38529: Make --ad-hoc the default for guix environment proposed deprecation mechanism

[Konrad Hinsen]

Hi Simon,

> Assuming "guix environment" would stay and following the proposed
> plan, you would need to add GUIX_ENVIRONMENT_DEPRECATED=1 on the top
> of your script. In this would not be a problem for travelling back in
> time.

The problem is not how I update my scripts - I can manage that, no
matter what it takes.

The problem is scripts circulating in public repositories, tutorials,
etc. New users will find them and use them for inspiration. It's very
discouraging to see examples from tutorials fail or do something weird.

The main precedent is the Python 2->3 transition. There are tons of
GitHub repositories with Python code but no indication if it's 2, 3, or
both. I even had to use one that executed with either 2 or 3, but gave
different results. It takes a lot of motivation to persist.

For guix, there's the additional issue that we use the reproducibility
of builds as an argument. Non-reproducible examples are then a bit of a
credibility problem.

Cheers,
  Konrad.