fix for adding multiple archives with same folders

when adding different (zip)archives with same folders, but different os constraints, an error is thrown.
example:
<file src="${packages.dir}/windows.zip" unpack="true" targetdir="$INSTALL_PATH/test">
                <os family="windows"/>
            </file>
            <file src="${packages.dir}//linux.zip" unpack="true" targetdir="$INSTALL_PATH/test">
                <os family="unix"/>
            </file>

in booth archives are the same folders and files, but for different operating systems.

during adding of the second zip, mkdirs() returns false, because the folder exists.