Automatically detect NixOS platform

[fricklerhandwerk]

Is your feature request related to a problem? Please describe.
Running the codelab on NixOS requires a manual step.

Describe the solution you'd like
Provide a wrapper for http_archive that additionally takes a mapping of platforms to a list of patches. See _detect_host_platform() in rules_nixpkgs for an example, or is_supported_platform() for a much simpler one.

Another way to detect NixOS: https://discourse.nixos.org/t/detect-that-bash-script-is-running-under-nixos/11402

Describe alternatives you've considered
Keep the manual step. Whoever uses NixOS will be confident enough to uncomment two lines in WORKSPACE.