[FIX] packaging: fix installing deb package when user exists

When a odoo user already exists, the installation of the deb package
fails because the `/var/lib/odoo` directory does not exists.

The reason is that the postinst script is trying to change the
permissions of this directory which is only created if a odoo user does
not already exists.

With this commit, the permission changes only occurs when the directory
is created.

closes odoo/odoo#111814

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>