preprocess: Image failing to unmount

[wasciutto]

Getting a cleanup error here causing the preprocess job to fail after fMRIPrep completes:

INFO: Cleanup error: while unmounting /var/apptainer/mnt/session/underlay/usr directory: device or resource busy, while unmounting /var/apptainer/mnt/session/final directory: device or resource busy

I have no idea if this is a longleaf issue or not. The issue happened in both 1.8 and 1.4 of clpipe. /work was added in 1.8, which is a likely culprit here, but it doesn't quite add up with 1.4 having the same issue. Maybe some sort of caching issue with Singularity caused by adding the work directory.