Motivation for this change

This PR changes the VirtualBox tests not to build the unfree tests by default.

The problem was that cache.nixos.org was running the tests, which caused the VirtualBox Oracle Extension Pack to be downloaded and cached. cache.nixos.org was then serving the Extension Pack to people when they tried to download it.

However, the license for the Extension Pack explicitly states that it cannot be served by third parties.

This PR changes the code for the VirtualBox test to not build and run the unfree tests by default. This causes cache.nixos.org to not build and cache the Extension Pack.

The VirtualBox tests can be run like the following:

$ cd nixos/tests
$ nix-build virtualbox.nix

This will only build and run the normal tests.

In order to build and run the unfree test, you must use the following command:

$ nix-build virtualbox.nix -A enable-extension-pack --arg enableUnfree true

If you try to run the enable-extension-pack test without setting the enableUnfree argument to true, you will get the following error:

$ nix-build virtualbox.nix -A enable-extension-pack
error: attribute 'enable-extension-pack' in selection path 'enable-extension-pack' not found

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.