Skip downloadToReadOnlyDir auto test on Windows

It should be rather skipped than blacklisted since it is not expected to
work on Windows.

See the corresponding warning about ACLs:
https://doc.qt.io/qt-5/qfile.html#setPermissions



Task-number: QTBUG-74698
Change-Id: Iad66c028583d6811b7a413dd9800e9095d8f6da7
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>