listrunner: fix agent usage

By delaying the agent key query until after the fork, we prevent the
problem of simultaneous access to the agent.

Tested that it works against 80 hosts in parallel without error; the
current version breaks already at 20 hosts.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>