Otherwise 'lua' is from the argument populated by callPackage
which means it's whatever the default is.

(instead of the lua we're generating packages for!)

Motivation for this change

Without this, I was finding lua53Packages managed to pull lua 5.2 along
with which caused confusion/mismatches.

This can be observed by specifying luaPackages = lua53Packages;
in top-level/all-packages.nix for the awesome package,
and inspect the build log (or try awesome --version) to see
what version of lua it detects.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.