s-tar: force '-std=c89' so it builds with recent compilers

[Moredread]

s-tar hasn't been building since the end of 2024, due to (I guess) a gcc update, that errors on more warnings. As star seems to target the old C standard, we enforce building it with -std=c89.

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.

---

Add a 👍 reaction to pull requests you find important.

[Moredread]

PR #357774 and #304628 are related to this, but I think we should fix this derivation, while the other PRs are still worked on.