build: added a nix flake

[teto]

I would like to help the community contribute to luarocks.org (with a few issues in mind see https://github.com/nvim-neorocks/luarocks-tag-release/discussions/73) . As it stands, testing changes seems like an obstacle:

• https://github.com/luarocks/luarocks-site/issues/176
• https://github.com/luarocks/luarocks-site/issues/150
• https://github.com/luarocks/luarocks-site/issues/157

which is why I would like to submit a nix package. nix is a package manager that focuses on reproducibility, basically what works on my machine works on yours. Many technologies promise this but nix delivers.

Here is my attempt at it (in my constrained time so anyone feel free to pick up).

• [x] dont use absolute location towards luajit in Makefile
• [ ] how to set an alternate postgresql port ?
• [x] add comment in nginx.conf about how variables are replaced
• [ ] improve instructions