[RAPPS] Fail to update database if "software source" is a read-only file://

It's quite a small issue and a rare use case:
I set the "software source" in rapps to be a local read-only file, `file:///E:/rappmgr2.cab`, where E: drive in virtualbox is a viso (virtual iso).

In rapps, I click the "update database" button. The first update is success. But, the rappmgr2.cab file is downloaded as read only. Thus, the second update fails with "Access is denied" when it attempts to clean up the existing file.

With the help of LLM, the issue is traced back to the `CopyFileW` call in `base/applications/rapps/loaddlg.cpp` -> `CDownloadManager::PerformDownloadAndInstall()`. It seems to keep the file's original attributes, affecting `file://` URIs.