build(nix): update dependencies

• Updated input 'fenix':
    'github:nix-community/fenix/eedfb5a' (2026-01-05)
  → 'github:nix-community/fenix/1ad578f' (2026-01-08)
• Updated input 'fenix/nixpkgs':
    'github:nixos/nixpkgs/fb7944c' (2026-01-02)
  → 'github:nixos/nixpkgs/5912c17' (2026-01-07)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/6a1246b' (2026-01-04)
  → 'github:rust-lang/rust-analyzer/54f5eea' (2026-01-07)

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Author: github-actions[bot]