Right now, it reads SYMFONY_DEPRECATIONS_HELPER using only getenv(), but this will soon become incompatible with Dotenv, since we switcher it to not use putenv() by default in new projects.

(note that I wouldn't change reading the other env vars in the class - only SYMFONY_DEPRECATIONS_HELPER makes sense for Dotenv)

/cc @greg0ire if you want to give it a try :)