workerpool: Don't keep reference to task arguments

The workerpool should not keep any reference to task arguments after
they were processed by RunTask. Doing so led to jobs being cached
by the job queue's WeakValueDictionary for longer than they should've
been.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Manuel Franceschini <livewire@google.com>