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/sigal/default.nix

@@ -1,4 +1,4 @@
-{ lib, python3Packages, ffmpeg }:
+{ lib, python3Packages, ffmpeg_3 }:
 
 python3Packages.buildPythonApplication rec {
   version = "2.0";
@@ -20,7 +20,7 @@ python3Packages.buildPythonApplication rec {
     blinker
   ];
 
-  makeWrapperArgs = [ "--prefix PATH : ${ffmpeg}/bin" ];
+  makeWrapperArgs = [ "--prefix PATH : ${ffmpeg_3}/bin" ];
 
   # No tests included
   doCheck = false;