Make --ad-hoc the default for guix environment proposed deprecation mechanism

[Gábor Boskovits]

Hello guix,

Based on discussion on IRC the following plan for deprecation might work.
Comments are welcome:

1.
Make guix environment aware of an environment variable:
GUIX_ENVIRONMENT_DEPRECATED_AD_HOC
if this is defined, then fall back to the current behaviour.

Motivation: this would enable most script users to bypass the problem,
while fixing them, but it makes the users aware that they are using a
deprecated feature.
At the same time this should come with a news entry, an any other
announcements should be made that we usually do, so that support don't
get overloaded. Is should also be announced that two releases later
the code supporting this will be removed, so that we don't have to
maintain it, but allow enough time for adoption.

2. add a flag to guix environment, something like
--ignore-deprecated-ad-hoc, that makes guix environment ignore the
environment variable, and default to the new behaviour.

Motivation: so that scripts can be fixed individually by modifying the
guix environment call to the new version, and adding the flag, so that
it does not cause a problem in the trasitional period while the
environment variable is defined.

3. on the specified release remove the environment variable support
code, and make the flag a noop, and also deprecated.

4. later if needed after an adoption period we can remove the flag.


Best regards,
g_bor
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