No PHP memory limit during build time

[EtienneM]

For instance we execute a PHP Symfony command cache:clear at the end of the deployment of a Symfony app. But at some point it may fail due to the lack of memory. We actually limit the PHP memory to 196 MiB at this point of the deployment. It is possible to customize it in the composer.json .extra.paas.php-config but we should probably stop limiting the memory here.

Reported by a customer here: https://app.intercom.com/a/apps/w4oogu7s/inbox/inbox/conversation/12375700069446