Also Use Signed-By: /etc/apt/keyrings/preinstalled-pool.gpg and
make sure we only update from that .sources file as we did before.
This code may all be dead, who can say.
FIXME: We should figure out how to do an armored export of that key
and then embed it in the signed-by field instead of using a keyring
file.
04d1f1af25