Fix for php session path not being writable in the cli

Noticed a bunch of session-related permission errors after updating an old PuPHPet VM and realized it was due to changes in how the session save path directory was handled.

The `Php.pp.twig` file used to create them with mode `0775` but that changed in 8e30179a5b8878a2440d69d91b22a797bc1f06f5, and now scripts being run as the `vagrant` user in the CLI (such as unit tests) can't write to the session dir.