Release process: drop the need for allocating (duplicated) temporary storage space for IUKs on rsync.lizard (refs: #17412)

Previously, we would copy the IUKs to /home and then move them to /srv/rsync.
Given these 2 directories are on different storage volumes, this means
we had to allocate the corresponding storage space twice.

Let's instead directly copy the IUKs to /srv.