rust.toRustTargetForUseInEnvVars: support custom targets

[alyssais]

Description of changes

If using a target spec JSON file, the <triple> value is the filename stem. For example --target foo/bar.json would match [target.bar].

- https://doc.rust-lang.org/cargo/reference/config.html#target

I've also exposed toRustTargetSpecShort as a public function, because it's useful to be able to know what the target subdirectory will be.

I think this is a minor regression in #247442. As these things go, I noticed just after merging it. >.>

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[RaitoBezarius]

Looks like a almost-perfect refactor for me.