add rustc and cargo to the cross trunk

[wucke13]

Compiling them can take hours, and with rust specifically it's quite common to cross compile (e.g. to musl/static for easier distribution)

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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

@Mindavi I'd be happy to get a review, I'm quite unsure about how this works

[yu-re-ka]

can't judge if this has any significant effects on hydra builds that should be considered
otherwise I'm very much in favor of this since I'm regularly building cross rustcs

[vcunat]

For future I think the "cross people" should consider some better approach for the built combinations than a single large matrix. For example, this PR added 2*65 jobs and I wonder how big a fraction will actually be useful. (You know, they'll all get fully rebuilt a few times each month.)