Show warning regarding PHP 8.4 support

[Jubeki]

The underlying library PHP-CS-Fixer is currently working on PHP 8.4 compatibility (this does not include PHP 8.4 syntax specific fixers).
See the following milestone for more information:
https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/milestone/173

It is therefor not recommended to run on PHP 8.4.

This PR implements a similar approach to was PHP-CS-Fixer does:

# With Pint:
PINT_IGNORE_ENV=1 vendor/bin/pint

# With PHP-CS-Fixer:
PHP_CS_FIXER_IGNORE_ENV=1 php php-cs-fixer.phar fix /path/to/dir

https://cs.symfony.com/doc/usage.html#environment-options

[Jubeki]

@crynobone what is your take on this?

And another question, what is the command for building the pint binary? (I can only find building the phar from the workflows)