Nightfall with ffmpeg >= v6.0

[niamu]

This changes the default static ffmpeg release to v6.1 currently to complement changes in Dusk-Labs/nightfall#15

This PR is in a draft until:

• it is known whether the nightfall dependency will receive an updated tag (so the temporary revision SHA in use can be replaced with a tag)
• the implications of no longer using Dusk-Labs/ffmpeg-static for the build are better understood
  • I attempted to update Dusk-Labs/ffmpeg-static myself, but it seems to need more attention than some version bumps in the build script