Publish channels to S3

[domenkozar]

I see three good reasons to do this:

• S3+cloudfront have been extremely stable (not that nixos.org was down for any long time) and they're decentralized, leaving last single point of failure on the side when using NixOS
• nix-channel --update would not redownload channel on each invocations since it would respect the etag header
• we won't need to recycle channels to free disk space, so declarative $NIX_PATH wouldn't fail anymore since upstream nixpkgs.tar.xz is missing

Would this add much to the costs?

cc @edolstra

[edolstra]

No, the only cost is having to rewrite the channel scripts.