Update homepage attributes: http -> https

Homepage link "http://.../" is a permanent redirect to "https://.../" and should be updated
https://repology.org/repository/nix_stable/problems

pkgs/development/libraries/gstreamer/libav/default.nix

@@ -4,7 +4,7 @@
 }:
 
 # Note that since gst-libav-1.6, libav is actually ffmpeg. See
-# http://gstreamer.freedesktop.org/releases/1.6/ for more info.
+# https://gstreamer.freedesktop.org/releases/1.6/ for more info.
 
 assert withSystemLibav -> libav != null;