treewide: use ffmpeg_3 explicitly if not wanted otherwise

After making `ffmpeg` point to the latest `ffmpeg_4`, all packages that
used `ffmpeg` without requiring a specific version now use ffmpeg_3
explicitly so they shouldn't change.

pkgs/applications/misc/moonlight-embedded/default.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchFromGitHub, cmake, perl
 , alsaLib, libevdev, libopus, udev, SDL2
-, ffmpeg, pkgconfig, xorg, libvdpau, libpulseaudio, libcec
+, ffmpeg_3, pkgconfig, xorg, libvdpau, libpulseaudio, libcec
 , curl, expat, avahi, enet, libuuid, libva
 }:
 
@@ -21,7 +21,7 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ cmake perl ];
   buildInputs = [
     alsaLib libevdev libopus udev SDL2
-    ffmpeg pkgconfig xorg.libxcb libvdpau libpulseaudio libcec
+    ffmpeg_3 pkgconfig xorg.libxcb libvdpau libpulseaudio libcec
     xorg.libpthreadstubs curl expat avahi enet libuuid libva
   ];