rustfmt: fix warning

[figsoda]

rustPlatform.rust has been deprecated in #230951

follow up to #231078

ofborg evals are failing because of the deprecation message

Description of changes

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/)
• 23.05 Release Notes (or backporting 22.11 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.

[winterqt]

What poor timing, definitely wasn't a good idea to ignore OfBorg 😄

[winterqt]

Waiting on eval, not taking any chances this time.

[mweinelt]

I think that's not going to cut it 🤡

[winterqt]

No clue what could be failing at this point.

Let's... try again? Surely it'll be different.
@ofborg eval

[figsoda]

ofborg-eval outputs all the rebuilds, so maybe it needs to eval both before and after this patch, and the warning could be from before this change

[winterqt]

Good point... let's do it.