haskell.compiler: use structured attrs for hadrianFlags

[sternenseemann]

Still needs changelog entry and testing.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[SFrijters]

Was just about to kick off a new __structuredAttrs = true; build so I'll add this into my branch for testing.

[sternenseemann]

I was able to build haskell.packages.ghc9103 and pkgsStatic.buildPackages.haskell.compiler.ghc9122 on x86_64-linux.

[SFrijters]

I used this PR instead of my own hack while rebuilding my system last week and it worked for everything I was doing. I also like just having __structuredAttrs enabled here.

[wolfgangwalther]

Still needs changelog entry

Since I have a big rebuild lined up on haskell-updates, I will merge this now. We can add the changelog entry later.

Edit: To be clear, I'm not sure whether we need one. This doesn't change the exposed interface, so is technically not a breaking change.