flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/e8361cc' (2025-12-06)
  → 'github:nix-community/fenix/16642c5' (2025-12-20)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/2cbf358' (2025-12-05)
  → 'github:rust-lang/rust-analyzer/ea1d299' (2025-12-18)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/f61125a' (2025-12-05)
  → 'github:NixOS/nixpkgs/c6245e8' (2025-12-18)
• Updated input 'rust-overlay':
    'github:oxalica/rust-overlay/086fd19' (2025-12-06)
  → 'github:oxalica/rust-overlay/66bb33f' (2025-12-20)

Author: github-actions[bot]