loader: lua: unload the kernel when changing BEs

Usually the kernel is loaded later, but there are circumstances where it
could have been loaded earlier than changing BEs. Unload anything that
is already there so that we know we're using artifacts from the proper
environment.

PR: 265471
Reviewed by: kevans
MFC after: 3 days

(cherry picked from commit d7584aa09fcf8d96544e5bfb3ea78f50d7720c0c)