build(flake): update flake.lock

[github-actions]

Automated changes by the update-flake-lock GitHub Action.

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/5d67ea6b4b63378b9c13be21e2ec9d1afc921713?narHash=sha256-Pj39hSoUA86ZePPF/UXiYHHM7hMIkios8TYG29kQT4g%3D' (2024-12-11)
  → 'github:NixOS/nixpkgs/20075955deac2583bb12f07151c2df830ef346b4?narHash=sha256-HMwfAJBdrr8wXAkbGhtcby1zGFvs%2BStOp19xNsbqdOg%3D' (2025-08-19)

Running GitHub Actions on this PR

GitHub Actions will not run workflows on pull requests which are opened by a GitHub Action.

To run GitHub Actions workflows on this PR, run:

git branch -D update_flake_lock_action
git fetch origin
git checkout update_flake_lock_action
git commit --amend --no-edit
git push origin update_flake_lock_action --force