Fixes #540.
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if (!systemTypes.count(step->drv.platform)) return false;
if (!systemTypes.count(step->drv.platform == "builtin" ? nix::settings.thisSystem : step->drv.platform)) return false;
if (!systemTypes.count(step->drv.platform == "builtin" ? nix::settings.thisSystem : step->drv.platform))
return false;