Automatically add apple virtualization to system-features

[roberth]

Is your feature request related to a problem? Please describe.

MacOS has two virtualization frameworks. By introducing these feature labels we allow for machines without such support to join a build farm.

Describe the solution you'd like

Add system-features = nixos-test to the default/example configuration and add "hvf" and "avf" dynamically based on whether these technologies are available, similar to how we do it for "kvm".

Describe alternatives you've considered

• Remove the requiredFeatures from Run nixosTests on darwin nixpkgs#193336 (comment)
  This will lead to problems when virtualized, non-standard or hardened darwin systems are part of a build cluster.

Additional context

Most MacOS systems support virtualization, but this is not a given.

AVF: https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_security_virtualization

nix/src/libstore/globals.cc

Lines 126 to 143 in fb8fc6f

	StringSet Settings::getDefaultSystemFeatures()
	{
	/* For backwards compatibility, accept some "features" that are
	used in Nixpkgs to route builds to certain machines but don't
	actually require anything special on the machines. */
	StringSet features{"nixos-test", "benchmark", "big-parallel"};
	#if __linux__
	features.insert("uid-range");
	#endif
	#if __linux__
	if (access("/dev/kvm", R_OK | W_OK) == 0)
	features.insert("kvm");
	#endif
	return features;
	}

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