Avoid problematic symlinking in download assets tasks of the web UI
We recently introduced support for download asset tasks when using
isos post. When there are multiple assets to be downloaded from the same URL these assets are only downloaded once and symlinks are used. The use of symlinks can be problematic because our cleanup might not treat them well, e.g. it might delete the symlink target but not the symlink itself leaving it broken.
We haven't seen this problem in practice yet so that's just a conclusion I came to when reading the download code. A possible solution would be using hard links with a fall back to copying (in case hard linking is not possible, e.g. because different filesystems are used).