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/python-modules/imageio/default.nix

@@ -8,7 +8,7 @@
 , pytest
 , numpy
 , isPy3k
-, ffmpeg
+, ffmpeg_3
 , futures
 , enum34
 }:
@@ -23,7 +23,7 @@ buildPythonPackage rec {
   };
 
   checkInputs = [ pytest psutil ] ++ stdenv.lib.optionals isPy3k [
-    imageio-ffmpeg ffmpeg
+    imageio-ffmpeg ffmpeg_3
     ];
   propagatedBuildInputs = [ numpy pillow ] ++ stdenv.lib.optionals (!isPy3k) [
     futures