coredump: make sure pid1/journal coredumps are not lost when Storage=journal is selec...

It confused the hell of me, that if pid1 crashes in an mkosi system run
from the build tree there's no coredump kept. Because mkosi configures
journal storage for coredumps, but that's not going to work for pid1 or
journald. Hence use external storage for these two even if everything
else is stored in the journal.