With this patch, we change the working dir to GlobalWorkDir at the AddJob
level. This way is easier to restore the old working dir, and we also avoid
some duplicated code between cliinterface and libarchiveplugin.

Note that the libarchive plugin used to add trailing slashes to relative folder
paths, while CliInterface didn't. Both plugins now receive relative folders
already with a trailing slash. This doesn't seem to change how
CliInterface::addFiles works.