build(nix): update dependencies

• Updated input 'fenix':
    'github:nix-community/fenix/fe00060' (2026-01-12)
  → 'github:nix-community/fenix/63cba4b' (2026-01-15)
• Updated input 'fenix/nixpkgs':
    'github:nixos/nixpkgs/ffbc9f8' (2026-01-11)
  → 'github:nixos/nixpkgs/1412caf' (2026-01-13)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/dd48777' (2026-01-11)
  → 'github:rust-lang/rust-analyzer/04efa1d' (2026-01-14)
• Updated input 'nixpkgs-unstable':
    'github:NixOS/nixpkgs/3fbab70' (2026-01-12)
  → 'github:NixOS/nixpkgs/13868c0' (2026-01-14)

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Author: github-actions[bot]