nix: Respect -I, --arg, --argstr

src/libexpr/common-eval-args.cc

src/libexpr/common-eval-args.hh

src/libexpr/common-opts.hh

src/libmain/common-args.cc

src/libmain/shared.cc

src/libmain/shared.hh

src/libstore/globals.cc

src/libutil/args.cc

src/libutil/args.hh

src/libutil/config.cc

src/nix-build/nix-build.cc

src/nix-env/nix-env.cc

src/nix-instantiate/nix-instantiate.cc

src/nix-prefetch-url/nix-prefetch-url.cc

src/nix/command.cc

src/nix/command.hh

src/nix/copy.cc

src/nix/hash.cc

src/nix/installables.cc

src/nix/main.cc