chromiumOSPackages.updateScript: fix buildspec url
chromiumOSPackages.updateScript: fix buildspec url

"paladin" doesn't seem to be up to date, whereas "full" seems to match
what cros-updates-serving.appspot.com reports is currently being
shipped to Chromebooks.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs