`download_file` shouldn't be destructive

Describe the Bug

The built-in download_file function is destructive and will remove existing files of the destination directory. This is a problem when trying to download multiple files from a target that would result in calling download_file twice.

This might not be considered a bug, but it is definitely undesirable behavior from my point of view.

Expected Behavior

Stop removing files. Overwrite behavior is probably fine, but I can imagine situations where that wouldn't be desirable. Perhaps this could be a config option.

Steps to Reproduce

Steps to reproduce the behavior:
From a single plan perform the following actions:

1. Call download_file('/src/foo.txt', 'foo.txt', $target)
2. Call download_file('/src/bar.txt', 'bar.txt', $target)
3. Inspect downloads/<target> to see that foo.txt has been deleted.

Environment

• Version 3.26.1
• Platform Ubuntu 20.04

Start by locating the built-in download_file entry point and reproduce the two-call sequence described in the issue against a target download directory. Confirm that downloading bar.txt no longer removes foo.txt, and add or update coverage for this behavior if the relevant test location is found.