nixosTests.virtualbox: fix eval

There's no "lib" in scope here. The test uses "with pkgs.lib;", so
this failed to evaluate.

Fixes: 6672dde558ea ("treewide: use optionalAttrs instead of 'else {}'")

Changed files
+1 -1
nixos
+1 -1
nixos/tests/virtualbox.nix
···
destroy_vm_test1()
destroy_vm_test2()
'';
-
} // (lib.optionalAttrs enableUnfree unfreeTests)
+
} // (optionalAttrs enableUnfree unfreeTests)