ansible: add validation/test to keep tasks from touching /mnt/{tmp,permanent}

/mnt/{tmp,permanent} aren't mounted when ansible runs (e.g. when building the container), and I keep forgetting this when writing new tasks... there must be some way to throw an error when the playbook is executing or ?? when a task tries to touch things in these directories... Martin had some ideas in the comment here: https://gitlab.freedesktop.org/mupuf/valve-infra/-/merge_requests/318#note_1539173