This does not build FFmpeg or implicitly link with any of its libraries. It's just the bindings.
First, update your build.zig.zon:
zig fetch --save https://github.com/myQwil/ffmpeg/archive/refs/heads/main.tar.gz
Next, add this snippet to your build.zig script:
const av = b.dependency("ffmpeg", .{}).module("av");
my_module.addImport("av", av);