Currently `extracttest` has random failures when we check properties
with the libarchive plugin.

This happens because there is a race condition between
`LoadJob::onFinished()` (where we read some `LoadJob` members) and
`LoadJob::onNewEntry()` (where we write those members).

`onFinished()` is called when `list()` returns, in `LoadJob::doWork()`.
But that might happen before `onNewEntry()` is called, since they are executed
in two different threads.

This patch puts the `onFinished()` call in the event queue, just like the
single-thread case does (where we emit the `finished` signal from `CliInterface`).

Differential Revision: D3111