Consider getting rid of `toolchain_sort()`

[rami3l]

While indeed the derived Ord instance are not used, I guess I was doing something idiomatic here (not entirely my fault, the function has been in existence way before #3518):

rustup/src/toolchain/names.rs

Lines 283 to 306 in f88394c

	/// Sorts [`ToolchainName`]s in the following order:
	/// 1. `stable`/`beta`/`nightly`-prefixed names, in this exact order.
	/// 2. `X.Y.Z-suffix` names, sorted by semver rules on `X.Y.Z`, then by `suffix`.
	/// 3. Other names, sorted alphanumerically.
	pub(crate) fn toolchain_sort(v: &mut [ToolchainName]) {
	v.sort_by_key(|name| {
	let s = name.to_string();
	if s.starts_with("stable") {
	return (0, None, s);
	}
	if s.starts_with("beta") {
	return (1, None, s);
	}
	if s.starts_with("nightly") {
	return (2, None, s);
	}
	if let Some((ver_str, suffix)) = s.split_once('-') {
	if let Ok(ver) = semver::Version::parse(ver_str) {
	return (3, Some(ver), suffix.to_owned());
	}
	}
	(4, None, s)
	})
	}

How about making use of that to write an actual Ord instance?

#3880 (comment)