rustup: 1.17.0 -> 1.18.1

[xrelkd]

Motivation for this change

Update rustup to the latest release.
https://github.com/rust-lang/rustup.rs/releases/tag/1.18.1
Changes: https://github.com/rust-lang/rustup.rs/blob/1.18.1/CHANGELOG.md

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[sondr3]

We could probably automatically generate shell completion when installing Rustup from this PR.

[xrelkd]

@sondr3 @Mic92 Thank you!