[Enhancement] pin luarocks version / manage luarocks installation

[mrcjkb]

Our installer currently builds luarocks from source (HEAD), which could be bad if something breaks.
It would be better for us to pin the version.

We could add a CI workflow that updates the pinned version and auto-merges if our checks succeed (we have enough test coverage to do that now).

Another idea would be for the installer to install luarocks into a temp directory (or to use the system installation if detected)
and for rocks.nvim to add luarocks to its dependencies.
This needs to be investigated and tested.