Change default export folder to location of opened file

### Your idea

When I open a file for editing (Right click > Open with > Audacity, and not drag-and-drop, if that makes a difference), and then try to export the edited audio to a file, it would be better if the default export folder is the location of the opened file instead of the previously used folder, or fixed default location, which is currently the only alternative.

I'm using Audacity 3.3.2 on Windows 10.

### Problem to be solved

Implementing this change would allow users not to have to specify the export folder every time they use the app if they happen not to store all their audio files in a single folder, which appears to be the expectation for the current design.

### Prior art

The behavior I suggest is typical of most text and image editing programs I have used.

### Additional context

_No response_