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/libraries/kde-frameworks/kfilemetadata/default.nix

@@ -1,7 +1,7 @@
 {
   mkDerivation, lib, copyPathsToStore,
   extra-cmake-modules,
-  attr, ebook_tools, exiv2, ffmpeg, karchive, kcoreaddons, ki18n, poppler, qtbase, qtmultimedia, taglib
+  attr, ebook_tools, exiv2, ffmpeg_3, karchive, kcoreaddons, ki18n, poppler, qtbase, qtmultimedia, taglib
 }:
 
 mkDerivation {
@@ -9,7 +9,7 @@ mkDerivation {
   meta = { maintainers = [ lib.maintainers.ttuegel ]; };
   nativeBuildInputs = [ extra-cmake-modules ];
   buildInputs = [
-    attr ebook_tools exiv2 ffmpeg karchive kcoreaddons ki18n poppler qtbase qtmultimedia
+    attr ebook_tools exiv2 ffmpeg_3 karchive kcoreaddons ki18n poppler qtbase qtmultimedia
     taglib
   ];
   patches = copyPathsToStore (lib.readPathsFromFile ./. ./series);