Overhaul build using luarocks

The atempts to manually install the bits ends up with no metadata to build a manifest from. This means luarocks list will not find this even though it is installed. Since upstream provides a rockspec (sadly not up to date in the tagged releases, it is always posted after the fact) lets just use it...

For an example of why this can be an issue see discussion here: https://github.com/sile-typesetter/sile/issues/2118