rustc: generate config.toml from attrset

[kiike]

Description of changes

This PR changes the way rustc is configured during bootstrapping. At the moment, the configuration is done by passing configureFlags to the configure, a Python script that adapts an example TOML file. This script is considered by upstream as a shim, and although it can pretty much set any arbitrary key-value pair using the --set flags, this is error-prone as the script doesn't use a proper TOML parser. For instance, we regularly encountered errors during writing (due to configure) or reading (bootstrap) with a target architecture with a dot in it (thumbv8m.main-none-eabi).

According to upstream, the true configuration interface for rustc is the config.toml. As such, in this PR I propose having a configuration attrset that is serialized into a config.toml file.

At first sight, this change can be considered superfluous. However I think having a more direct interface to the config.toml file can be a better tool to avoid errors and ultimately to increase reproducibility, as the generated file is the true configuration source.

Things done

Right now I tested this with a cross-compilation for a nitrokey-3 firmware.

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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.

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