flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/e8361cc' (2025-12-06)
  → 'github:nix-community/fenix/e2b0f06' (2026-01-03)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/2cbf358' (2025-12-05)
  → 'github:rust-lang/rust-analyzer/e19dfc8' (2026-01-02)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/f61125a' (2025-12-05)
  → 'github:NixOS/nixpkgs/fb7944c' (2026-01-02)
• Updated input 'rust-overlay':
    'github:oxalica/rust-overlay/086fd19' (2025-12-06)
  → 'github:oxalica/rust-overlay/2941983' (2026-01-03)

Author: github-actions[bot]