Fix downloads report

[becdetat]

This is a copy of #5321, a PR from @sultann. I've copied it to:

• get CI building the branch
• change prepare_item_for_export to use $export_item to match other controllers

Note that I've tested @sultann's PR, this just needs approval for merging.

Detailed test instructions:

• Unless you have several hundred downloads this will be impossible to test
• The easiest way to see what happens with the typo is to introduce it to /src/API/Reports/Orders/Controller.php and run a orders download. The error log should fill up with errors and the downloaded report (emailed) will be full of empty lines.