crate2nix is now available in Nixpkgs, so we use it from there, allowing us to drop our workaround preventing the dependency upon LLVM, etc.

This means all supported systems have a crate2nix binary, without needing to depend on the pinned Nixpkgs from crate2nix’s repo!

Tested, thanks! crate2nix in Nixpkgs is great news. Even though I also use a number of ways of compiling Rust on Nix now (especially for the Nest, which seems too big for crate2nix and causes nix to segfault), this way of doing it is great and has its place.