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/ha-ffmpeg/default.nix

@@ -1,5 +1,5 @@
 { stdenv, buildPythonPackage, fetchPypi, isPy3k
-, ffmpeg, async-timeout }:
+, ffmpeg_3, async-timeout }:
 
 buildPythonPackage rec {
   pname = "ha-ffmpeg";
@@ -12,7 +12,7 @@ buildPythonPackage rec {
     sha256 = "230f2fa990c9caaff1c67c2227b64756062248083849651a9bec7d599e519a42";
   };
 
-  buildInputs = [ ffmpeg ];
+  buildInputs = [ ffmpeg_3 ];
 
   propagatedBuildInputs = [ async-timeout ];