⚡ Optimize unstable nixpkgs instantiation

[whitestrake]

💡 What:

• Hoisted the instantiation of nixpkgs-unstable out of common/all-systems.nix (which runs for every host) into flake.nix (which runs once per system architecture).
• Passed the instantiated unstable package set to modules via specialArgs.
• Updated common/all-systems.nix to use the injected unstable argument in the overlay.

🎯 Why:

• Previously, nixpkgs-unstable was imported and instantiated separately for every single host configuration. This is inefficient as it forces the evaluation of the entire unstable package set multiple times (once for each host of the same architecture).
• By instantiating it once in flake.nix and sharing it, we reduce redundant evaluation work.

📊 Measured Improvement:

• Benchmark: time nix eval .#checks.x86_64-linux --apply 'x: map (n: x.${n}.drvPath) (builtins.attrNames x)'
• Baseline: ~1m11s (user time ~1m16s)
• Optimized: ~1m2s - 1m13s (user time ~1m3s - 1m15s)
• The benchmark results on the available environment were mixed/inconclusive regarding raw time, likely due to other bottlenecks or caching, but the change is structurally strictly more efficient and prevents O(N) instantiation of the unstable package set.

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PR created automatically by Jules for task 7694995513334571880 started by @whitestrake

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