Fetch from the nixpkgs repo instead of nixpkgs-channels

nixpkgs-channels is deprecated.

helper/fetch-channel

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
 rev=$1
-sha256=$(nix-prefetch-url --unpack https://github.com/nixos/nixpkgs-channels/archive/$rev.tar.gz)
+sha256=$(nix-prefetch-url --unpack https://github.com/nixos/nixpkgs/archive/$rev.tar.gz)
 echo "rev = \"$rev\";"
 echo "sha256 = \"$sha256\";"