bug#79714: env: always adds __CF_USER_TEXT_ENCODING var on macOS since v9.8

On 2025-11-02 01:24, Bruno Haible via GNU coreutils Bug Reports wrote:
> We could document that users should not use environment variables
> that start with "__"

In addition to Pádraig's qualms, POSIX says[1] that if an environment 
variable's name consists solely of uppercase ASCII letters, '_', and 
non-initial digits, the variable might affect the behavior of standard 
utilities like 'cat'. So if we document environment variable gotchas, we 
should document that one too.

[1]: 
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html#tag_08_01