editors/neovim: Partially revert 9b869a4c17db42729cdfaca02c5c9c5ab2d22976

luajit-openresty causes lua errors with many nvim plugins. I don't know
why, and until I have time to diagnose it, revert the switch to
hardcoded luajit-openresty. In the short term I'll likely switch to
hardcoding luajit and removing the PUC Lua option.