This PR adds seeds a new environment variable named SYMFONY_PROCESS_IDENTIFIER to all created processes.

What is the motivation?

If you spawn multiple sub processes, you might want something that uniquely differentiates them.
To give an example, I like to refer to the Symfony messenger, which is a component usually utilizing multiple php processes / or workers (they might be managed by supervisord or from the main app, which is the case for Contao CMS; also see contao/contao#8527)

The Symfony docs states:

There should never be more than one messenger:consume command running with the same combination of stream, group and consumer, or messages could end up being handled more than once. If you run multiple queue workers, consumer can be set to an environment variable, like %env(MESSENGER_CONSUMER_NAME)%, set by Supervisor (example below) or any other service used to manage the worker processes.

We think, this is a universal requirement and might be considered added to Symfony.