Simplifying adding Cargo's bin path for fish so it's not version dependent

For the first shell, installation of rust ends with needing to set up a path variable for Cargo.

See examples in

- https://github.com/rust-lang/rustlings/issues/1533
- (Sorry for the weird link) https://web.archive.org/web/20241206023257/https://tecposter.com/posts/install-rust-via-asdf/

The path provided by installation script depends on a particular version of rust. Is there a way to handle this in a manner that's version agnostic?

Thanks!

Start by tracing the Rust installation script and the first-shell PATH setup for Cargo, then compare the linked Rustlings and asdf installation examples. Done means the fish Cargo path no longer depends on a particular Rust version and the installation flow clearly reflects the version-agnostic location.

Written by the indexing model from the issue text.