TranscodeAudioFileJob: Replace pipe with tempfile to prevent deadlock

[ntrrgc]

Using a regular file instead of a pipe prevents ffmpeg from blocking indefinitely waiting for the euphony to read -- which it never does until the ffmpeg subprocess exits.

Fixes #1

[simongoricar]

Oops, don't mind the CI fail, I cancelled it because I forgot to update runs-on to use ubuntu-latest instead of self-hosted that I was experimenting with.

[simongoricar]

Hi, I thought about it and looked into it a bit, and it turns out that we can still avoid creating temporary files by using anonymous pipes (see std::io::pipe).

But I think we can still merge this fix for a start until that's done, and we can look into improvements for this on the linked issue.