Considering --arg instead of --extra-nixpkgs-config

[SomeoneSerge]

The --extra-nixpkgs-config flag introduced in #315 has been very useful for our CUDA stuff: it lets one review variants of nixpkgs with { config.cudaSupport = true; } and even with custom { config.cudaCapabilities = [ "8.6" ]; }. It is limited in that e.g. it won't help testing nixpkgs variants with a custom cudatoolkit release, the like of import nixpkgs { ...; overlays = [ (final: prev: { cudaPackages = final.cudaPackages_XX_Y; }) ]; }. Additionally, --arg/--argstr allow to pass crossSystem

The --arg parameter could be introduced similarly to --extra-nixpkgs-config: the generated markdown report would simply include its text as is, thus satisfying that other reviewers always know how to run nixpkgs-review to get the same results.

Naturally, with --arg we wouldn't need --extra-nixpkgs-config anymore.

An extra benefit is that --arg is already familiar to users of nix3-eval, nix build -f, and nix-build.

The downside is that it's harder to validate the value of --arg, than the value of --extra-nixpkgs-config