flake.lock: Update

Flake lock file updates:

• Updated input 'fenix':
    'github:nix-community/fenix/dfaf3a97fe595a18bf9d55cda3416c17fe906f0f' (2023-11-08)
  → 'github:nix-community/fenix/8001e62f21df863304287c63f01fcc0c24c91816' (2023-12-16)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/3b7c7f97e4a7bb253a8d398ee4f8346f6cf2817b' (2023-11-07)
  → 'github:rust-lang/rust-analyzer/21b06c1beb9bb59369ffd652f5d617bcf6952e05' (2023-12-15)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/ec750fd01963ab6b20ee1f0cb488754e8036d89d' (2023-11-07)
  → 'github:NixOS/nixpkgs/aa9d4729cbc99dabacb50e3994dcefb3ea0f7447' (2023-12-14)
• Updated input 'utils':
    'github:numtide/flake-utils/ff7b65b44d01cf9ba6a71320833626af21126384' (2023-09-12)
  → 'github:numtide/flake-utils/4022d587cbbfd70fe950c1e2083a02621806a725' (2023-12-04)