tealdeer: fix tests on darwin

[ryan4yin]

Resolves #462177.

tealdeer 1.8.x now requires run tests with --test-threads 1: https://github.com/tealdeer-rs/tealdeer/blob/v1.8.1/.github/workflows/ci.yml#L46-L47

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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[ryan4yin]

nixpkgs-review result

Generated using nixpkgs-review-gha

Command: nixpkgs-review pr 462776

Logs: https://github.com/ryan4yin/nixpkgs-review-gha/actions/runs/19454583431/job/55665965621

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aarch64-linux (sandbox = true)

✅ 1 package built:

• tealdeer

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x86_64-linux (sandbox = true)

✅ 1 package built:

• tealdeer

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aarch64-darwin (sandbox = true)

✅ 1 package built:

• tealdeer