build: update flake + add neovim-with-rocks derivation for testing

[mrcjkb]

This allows to manually test rocks-dev.nvim using nix with

nix run .#neovim-with-rocks

It currently errors at startup because of the --force-lock flag. I'm not sure why. It should be using the rocks.nvim luarocks 3.11.1 package.

[github-actions[bot]]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(installer): some installer bugfix

• [ ] Pull request title has the appropriate conventional commit prefix.

If applicable:

• [ ] Tested
  • [ ] Tests have been added.
  • [ ] Tested manually (steps in PR description).
• [ ] Updated documentation.