Channels have moved from `nixos.org/channels` to `cnannels.nixos.org`.
This udates all relevant links (excluding release notes) to use the new
canonical URLs and replaces HTTP with HTTPS.
Fixes#1031.
Change-Id: I212821c44ac5e482c8e9eaa415c7d8ee17ff8341
Signed-off-by: adam <me@adamperkowski.dev>
