flake.lock: update nixpkgs

[a-kenji-buildbot]

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

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/63c3a29ca82437c87573e4c6919b09a24ea61b0f' (2024-05-02)
  → 'github:nixos/nixpkgs/e9be42459999a253a9f92559b1f5b72e1b44c13d' (2024-05-03)

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