vlc: pin ffmpeg to 4.x to fix VAAPI

It turns out that https://github.com/NixOS/nixpkgs/pull/343567 did not
go far enough. It appears that in fact, this is an upstream bug that
Fedora people found:
https://discussion.fedoraproject.org/t/vlc-not-exposing-va-api-as-a-hardware-acceleration-option/101133
Upstream bug in VLC: https://code.videolan.org/videolan/vlc/-/issues/26772

I don't like this change. However, I have tested it, and it does fix
VAAPI on my machine, making it reappear in the menu as an acceleration
option.

 » result/bin/vlc ~/test.mp4
VLC media player 3.0.21 Vetinari (revision 3.0.21-0-gdd8bfdbabe8)
[000000002efd0520] main libvlc: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
[00007f35c4004e00] gl gl: Initialized libplacebo v5.264.1 (API v264)
libva info: VA-API version 1.22.0
libva info: Trying to open /run/opengl-driver/lib/dri/iHD_drv_video.so
libva info: Found init function __vaDriverInit_1_22
libva info: va_openDriver() returns 0
[00007f35dcc1b710] avcodec decoder: Using Intel iHD driver for Intel(R) Gen Graphics - 24.3.4 () for hardware decoding

pkgs/by-name/vl/vlc/package.nix

@@ -12,7 +12,11 @@
   fetchpatch,
   fetchurl,
   # Please unpin FFmpeg on the next upstream release.
-  ffmpeg_6,
+  # Currently FFmpeg is pinned to 4.x because VAAPI acceleration is broken when
+  # building with newer versions:
+  # https://code.videolan.org/videolan/vlc/-/issues/26772
+  # This is intentional by upstream but VLC 4.0 will support newer FFmpeg.
+  ffmpeg_4,
   flac,
   fluidsynth,
   freefont_ttf,
@@ -139,7 +143,7 @@ stdenv.mkDerivation (finalAttrs: {
       avahi
       dbus
       faad2
-      ffmpeg_6
+      ffmpeg_4
       flac
       fluidsynth
       fribidi