feat: update flake.lock (#96)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/fbcf476f790d8a217c3eab4e12033dc4a0f6d23c?narHash=sha256-wNO3%2BKs2jZJ4nTHMuks%2BcxAiVBGNuEBXsT29Bz6HASo%3D' (2025-08-14)
  → 'github:nixos/nixpkgs/20075955deac2583bb12f07151c2df830ef346b4?narHash=sha256-HMwfAJBdrr8wXAkbGhtcby1zGFvs%2BStOp19xNsbqdOg%3D' (2025-08-19)

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Author: github-actions[bot]