[placeholder] Always export `FRACTAL_CACHE_DIR`

[tcompa]

The idea would be that FRACTAL_CACHE_DIR is always available when tasks run (note: this should hold for local/sudo/ssh deployments - TBD) - so that task developers can rely on this folder from within tasks.

Requirements:

• Folder is writeable for all (machine) users who can run the task

[jluethi]

Folder is writeable for all (machine) users who can run the task

To achieve this, one idea is setting it user-specific to {user.project_dir}/.fractal_cache

Whatever account runs jobs for the user anyway needs write access to {user.project_dir}, as output Zarrs are written their by default.