flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/e8361cc' (2025-12-06)
  → 'github:nix-community/fenix/6399553' (2025-12-11)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/2cbf358' (2025-12-05)
  → 'github:rust-lang/rust-analyzer/fface27' (2025-12-10)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/f61125a' (2025-12-05)
  → 'github:NixOS/nixpkgs/2fbfb1d' (2025-12-11)
• Updated input 'rust-overlay':
    'github:oxalica/rust-overlay/086fd19' (2025-12-06)
  → 'github:oxalica/rust-overlay/348b94e' (2025-12-13)

Author: github-actions[bot]