Refactor lz4 override to shared package

[whitestrake]

This PR refactors the inline lz4 override found in hosts/omnius/default.nix into a standalone package definition in pkgs/lz4.nix.

Changes

• Moved: The inline postInstall logic that creates the lz4c symlink is now in pkgs/lz4.nix.
• Registered: The new package is exposed via pkgs/default.nix as lz4 (accessible via myPkgs.lz4).
• Updated: hosts/omnius/default.nix now uses the shared package instead of an inline override.

Benefits

• Deduplication: The derivation can now be shared/cached properly.
• Maintainability: Package logic is centralized in pkgs/.

---

PR created automatically by Jules for task 16296258148251996757 started by @whitestrake

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.