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/tools/nwjs/default.nix

@@ -3,7 +3,7 @@
 , xorg, alsaLib, dbus, glib, gtk3, atk, pango, freetype, fontconfig
 , gdk-pixbuf, cairo, nss, nspr, gconf, expat, systemd, libcap
 , libnotify
-, ffmpeg, libxcb, cups
+, ffmpeg_3, libxcb, cups
 , sqlite, udev
 , libuuid
 , sdk ? false
@@ -22,7 +22,7 @@ let
       xorg.libXScrnSaver cups
       libcap libnotify
       # libnw-specific (not chromium dependencies)
-      ffmpeg libxcb
+      ffmpeg_3 libxcb
       # chromium runtime deps (dlopen’d)
       sqlite udev
       libuuid