Add COMPOSER_MEMORY_LIMIT support to Symfony\Flex\ScriptExecutor

[PabloKowalczyk]

Hello, this change will allow to define same memory limit for Composer and sub-processes run by ScriptExecutor. Any interest in this?
I can provide PR if you are interested.

[nicolas-grekas]

Please send a PR if you are interested (that's how OSS work ;) )