flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/25db7a6' (2025-07-26)
  → 'github:nix-community/fenix/16642c5' (2025-12-20)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/51e77c9' (2025-07-25)
  → 'github:rust-lang/rust-analyzer/ea1d299' (2025-12-18)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/6027c30' (2025-07-25)
  → 'github:NixOS/nixpkgs/7d853e5' (2025-12-19)

Author: github-actions[bot]