Use File.createTempFile() when downloading files from a remote repository

Use File.createTempFile() when downloading artifacts from a remote repository during a proxy request.

Currently when a file is downloaded it is saved in a temporary location before being moved in place. If a second request for the same artifact is made it will try and write to the same location as the first request at the same time. This causes the repository server to return an error.

This is common in use cases where Archiva is proxying a large file from a slow connecton.