They will show up in machineTypes as (e.g.) x86_64-linux:local instead of x86_64-linux. This is to prevent the Hydra provisioner from creating machines for steps that are supposed to be executed locally.
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if (i != step->drv.env.end()) {
step->requiredSystemFeatures = tokenizeString<std::set<std::string>>(i->second);
StringSet features;
if (i != step->drv.env.end())
features = step->requiredSystemFeatures = tokenizeString<std::set<std::string>>(i->second);
if (step->preferLocalBuild)
features.insert("local");
if (!features.empty()) {