yazi: use ffmpeg-headless for video previews

The bundled video previewer only invokes `ffmpeg` and `ffprobe`
(yazi-plugin/preset/plugins/video.lua), never `ffplay`. The full
ffmpeg's `ffplay` links SDL, which transitively pulls in gtk3, gtk4,
zenity, gst-plugins-bad and flite.

Closure: 1.1G -> 494M.

Matches what comparable in-tree thumbnailers already do
(ffmpegthumbnailer, mcat, yt-dlp, navidrome).

pkgs/by-name/ya/yazi/package.nix

@@ -9,7 +9,7 @@
     jq
     poppler-utils
     _7zz
-    ffmpeg
+    ffmpeg-headless
     fd
     ripgrep
     fzf
@@ -27,7 +27,7 @@
   jq,
   poppler-utils,
   _7zz,
-  ffmpeg,
+  ffmpeg-headless,
   fd,
   ripgrep,
   fzf,