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/development/libraries/pipewire/0.2.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchFromGitHub, meson, ninja, pkgconfig, doxygen, graphviz, valgrind
-, glib, dbus, gst_all_1, alsaLib, ffmpeg, libjack2, udev, libva, xorg
+, glib, dbus, gst_all_1, alsaLib, ffmpeg_3, libjack2, udev, libva, xorg
 , sbc, SDL2, makeFontsConf
 }:
 
@@ -25,7 +25,7 @@ in stdenv.mkDerivation rec {
   ];
   buildInputs = [
     glib dbus gst_all_1.gst-plugins-base gst_all_1.gstreamer
-    alsaLib ffmpeg libjack2 udev libva xorg.libX11 sbc SDL2
+    alsaLib ffmpeg_3 libjack2 udev libva xorg.libX11 sbc SDL2
   ];
 
   mesonFlags = [