Fixed incorrect index in filenames when converting videos to audio

[cristianst85]

When converting playlist videos to audio the index in the filename was arbitrarily assigned (due to variable reuse in async tasks). This was regardless of the maximum concurrent file conversions setting.

URL used for testing: https://www.youtube.com/watch?v=uFLoZ-gr4PE&list=OLAK5uy_nLwrcZJLKqwkwW1k3GLuic1hA9NUchja8&index=5

YoutubePlaylistDownloader settings to reproduce the issue:

• File name pattern: $index $title
• Convert videos to mp3 and set the bit rate to 320 kbps
• Set the maximum concurrent file conversions to 1.

Before the fix (maximum concurrent file conversions was set to 1) https://github.com/shaked6540/YoutubePlaylistDownloader/assets/12585988/fcdf3100-d700-425c-af7c-c65c58e74f60

After the fix (maximum concurrent file conversions was set to 1) https://github.com/shaked6540/YoutubePlaylistDownloader/assets/12585988/1c147f6f-720c-4413-8b57-9ff6272da6a8

After the fix (maximum concurrent file conversions was set to 3) https://github.com/shaked6540/YoutubePlaylistDownloader/assets/12585988/7103ff03-bdba-47b0-9dab-d8add0a1da7c