mlt: Use selected version of ffmpeg for all dependencies

Right now if overriding ffmpeg (example `mlt.override { ffmpeg = ffmpeg-full; }`) will have unusual results as multiple versions of ffmpeg will end up linked into the resulting executable. I don't think it is well defined which will end up being used.

Notably for kdenlive which tries to use `ffmpeg-full` libaom-av1 was not available as the non-full ffmpeg was being used.

I don't know if this fix is the most maintainable solution, but it solves a particular problem and doesn't harm maintainability. It is likely wise to consider how we can manage full-binary library overrides better in the future.

Fixes: https://github.com/NixOS/nixpkgs/issues/272526