cliinterface: restore old working directory after extraction

Otherwise Ark might prevent unmounting of removable devices.

To fix this issue we only need to save the current working dir before
the very first `QDir::setCurrent()` call in `extractFiles()`, and also
to restore it at the end of every extraction.

However, since `m_oldWorkingDir` is used also by copy and move jobs, we
can't touch it from extract jobs, otherwise we'll break `movetest`.

As workaround, we can use a new variable that will be used only by
extract jobs.

BUG: 395939
FIXED-IN: 18.07.80