fix FTBFS: don't remove, just hold
Closes #19290 (closed)
removing the kernel is not good: the build will later fail with
INFO:__main__:Mounting ISO on /tmp/tmpq5tzp0op
INFO:__main__:Executing 'mount -o loop,ro tails-amd64-stable@e5b6634b3b-20221117T1052Z.iso /tmp/tmpq5tzp0op'
mount: /tmp/tmpq5tzp0op: unknown filesystem type 'iso9660'.
and we can expect it to be a source of more problems.
Just holding is more to the point.