apt-key: kill gpg-agent explicitly in cleanup

apt-key has (usually) no secret key material so it doesn't really need
the agent at all, but newer gpgs insist on starting it anyhow. The
agents die off rather quickly after the underlying home-directory is
cleaned up, but that is still not fast enough for tools like sbuild
which want to unmount but can't as the agent is still hanging onto a
non-existent homedir.

Reported-By: Johannes 'josch' Schauer on IRC