failure in tails-create-iuk causes extremely large output to stderr
If you mistakenly switched python-xattr for python-pyxattr (and I assume other kinds of errors may also cause this behaviour), creating IUK's fails with an excessive barrage or error messages about read-only filesystems. So much that it completely fills reasonable terminal buffers and makes it hard to figure out what actually caused the problem. For a command that's taking it's sweet time to run that's pretty annoying.
I'm not sure if this is easy to fix in the perl code (I assume there's some cleanup routine assuming squashfs mounts are already unmounted?). If not, we may want to change the release process documentation and add a tee catching stderr while doing the iuk creation, so at least you'll have a file where you can see the more useful error messaging at the beginning.