`cargo-wasix` assumes presence of `rustup`

[knarkzel]

$ cargo wasix build
error: failed to create process "rustup" "toolchain" "list" "--verbose"

Caused by:
    No such file or directory (os error 2)

This is annoying for NixOS users since we typically don't use rustup for managing the Rust toolchain but Nix instead. What should happen instead is that cargo-wasix should try to build, and if it fails to build display a helpful message instead of checking if the toolchain is available beforehand. Message can be something like this:

Toolchain not found, make sure to install it

[knarkzel]

It seems like rustup is necessary due to wasix being an experimental target, I'll just use rustup in the mean time.