Keep track of requiredSystemFeatures in the machine stats

For example, steps that require the "kvm" feature may require a
different kind of machine to be provisioned. This can also be used to
require performance-sensitive tests to run on a particular kind of
machine, e.g., by setting requiredSystemFeatures to something like
"ec2-i2.8xlarge".

src/hydra-queue-runner/state.hh

@@ -135,6 +135,7 @@ struct Step
     nix::Derivation drv;
     std::set<std::string> requiredSystemFeatures;
     bool preferLocalBuild;
+    std::string systemType; // concatenation of drv.platform and requiredSystemFeatures
 
     struct State
     {