Motivation for this change

Don't figure modules directory in builder, pass it as variable from nix expression instead.

We need this for nvidia optimus script to work on non-NixOS:
https://nixos.wiki/wiki/nvidia#Nvidia_PRIME
dukzcry/crap#3 (comment)

We override nix packages in script so modules could be built against kernel on non-NixOS:
https://github.com/dukzcry/crap/blob/master/primerun/primerun.sh#L187

cast @crzyBrown (script user), @vcunat (nvidia package maintainer)

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)
• Fits CONTRIBUTING.md.