flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/c3c27e603b0d9b5aac8a16236586696338856fbb' (2025-05-16)
  → 'github:nix-community/fenix/381579dee168d5ced412e2990e9637ecc7cf1c5d' (2025-12-21)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/f8e784353bde7cbf9a9046285c1caf41ac484ebe' (2025-05-15)
  → 'github:rust-lang/rust-analyzer/765b7bdb432b3740f2d564afccfae831d5a972e4' (2025-12-20)
• Updated input 'gitignore':
    'github:hercules-ci/gitignore.nix/637db329424fd7e46cf4185293b9cc8c88c95394' (2024-02-28)
  → 'github:hercules-ci/gitignore.nix/cb5e3fdca1de58ccbc3ef53de65bd372b48f567c' (2025-11-10)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/063f43f2dbdef86376cc29ad646c45c46e93234c' (2025-05-23)
  → 'github:NixOS/nixpkgs/c6245e83d836d0433170a16eb185cefe0572f8b8' (2025-12-18)

Author: github-actions[bot]