Use top-level cctools when available

[reckenrode]

cctools was updated and migrated to the by-name hierarchy in nixpkgs, which moves it to the top-level. It is also being added to darwin-aliases.nix, which will make the old name unavailable for use in nixpkgs. This change preferentially uses the new name while falling back to the old one for out-of-tree users.

Relevant nixpkgs PRs:

• https://github.com/NixOS/nixpkgs/pull/307880
• https://github.com/NixOS/nixpkgs/pull/328077