GitHub Action to update Nix Flake lock

[peterbecich]

using https://github.com/DeterminateSystems/update-flake-lock

[peterbecich]

@gbaz , would you consider enabling "Allow GitHub Actions to create and approve pull requests"?

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#preventing-github-actions-from-creating-or-approving-pull-requests

This will enable the Nix Flake lock to be automatically updated (though still requiring manual merge of the PR) https://github.com/haskell/hackage-server/actions/runs/12524617063/job/34935330082