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/thumbor/default.nix

@@ -25,7 +25,7 @@ buildPythonPackage rec {
       gifsicle = "${pkgs.gifsicle}/bin/gifsicle";
       exiftool = "${pkgs.exiftool}/bin/exiftool";
       jpegtran = "${pkgs.libjpeg}/bin/jpegtran";
-      ffmpeg = "${pkgs.ffmpeg}/bin/ffmpeg";
+      ffmpeg = "${pkgs.ffmpeg_3}/bin/ffmpeg";
     })
   ];