vagrant.dev: optimise composer cache for comptoir user

Steps to reproduce

vagrant up
vagrant ssh
sudo su comptoir; 
cd /home/comptoir/Comptoir-srv
rm -rvf vendor/
composer install

What is the current bug behavior?

The first run off composer install for comptoir user don't use composer cache.

What is the expected correct behavior?

The first run off composer install for comptoir user must use composer cache.