Task processing logs should record apptainer overlays being used

[prioux]

When a CbrainTask is launched, the associated ToolConfig object can specify a list of overlays to mount within the apptainer container. This list is maintained in the attribute singularity_overlays_specs. During the Setting Up stage of the task, these overlays are fetched (if needed) to cache and their paths are added to the apptainer run command.

However, the task's log has no record of these paths begin set up, because the work is performed by a method in the ToolConfig class (method singularity_overlays_full_paths).

I'd like the code to be adjusted so that the task's setup log says explicitely which files are mounted and how they were fetched (direct configuration with a path, or through a userfile, etc etc).