[Feature request] Ability to override "temporary conversion directory"

[alexindigo]

Idea

Would be great to have ability to override "temporary conversion directory".

For example, in my case, I'd like to have temporary conversion folder outside of the output folder, I want to use local harddrive for temporary files, while final output is on mounted NFS drive.

Would it be possible to add environment variable to override default temporary conversion directory?

Thank you.