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/misc/emulators/ppsspp/default.nix

@@ -1,7 +1,7 @@
 { SDL2
 , cmake
 , fetchFromGitHub
-, ffmpeg
+, ffmpeg_3
 , glew
 , lib
 , libzip
@@ -37,7 +37,7 @@ mkDerivation rec {
 
   buildInputs = [
     SDL2
-    ffmpeg
+    ffmpeg_3
     glew
     libzip
     qtbase