build(nix): update dependencies

• Updated input 'fenix':
    'github:nix-community/fenix/fe00060' (2026-01-12)
  → 'github:nix-community/fenix/f6d71e5' (2026-01-14)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/dd48777' (2026-01-11)
  → 'github:rust-lang/rust-analyzer/3e8fc50' (2026-01-13)
• Updated input 'nixpkgs-unstable':
    'github:NixOS/nixpkgs/3fbab70' (2026-01-12)
  → 'github:NixOS/nixpkgs/ea30586' (2026-01-14)

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Author: github-actions[bot]