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/tools/graphics/cfdg/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, libpng, bison, flex, ffmpeg, icu }:
+{ stdenv, fetchFromGitHub, libpng, bison, flex, ffmpeg_3, icu }:
 
 stdenv.mkDerivation rec {
   pname = "cfdg";
@@ -10,7 +10,7 @@ stdenv.mkDerivation rec {
     sha256 = "13m8npccacmgxbs4il45zw53dskjh53ngv2nxahwqw8shjrws4mh";
   };
 
-  buildInputs = [ libpng bison flex ffmpeg icu ];
+  buildInputs = [ libpng bison flex ffmpeg_3 icu ];
 
   postPatch = ''
     sed -e "/YY_NO_UNISTD/a#include <stdio.h>" -i src-common/cfdg.l