flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/f7c6852faf89408d2f879ba421204376f9e333cc' (2025-08-03)
  → 'github:nix-community/fenix/381579dee168d5ced412e2990e9637ecc7cf1c5d' (2025-12-21)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/5daac5567bab99ce3819b666d80545cf7df66c18' (2025-08-02)
  → '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/e960e6e1dc689e2b5fe9e3ee824fc12860d9f27c' (2025-08-03)
  → 'github:nixos/nixpkgs/7d853e518814cca2a657b72eeba67ae20ebf7059' (2025-12-19)

Author: github-actions[bot]