Don't set $preferLocalBuild and $requiredSystemFeatures in builders

With C++ std::map, doing a comparison like ‘map["foo"] == ...’ has the
side-effect of adding a mapping from "foo" to the empty string if
"foo" doesn't exist in the map.  So we ended up setting some
environment variables by accident.

src/libstore/misc.hh

@@ -32,7 +32,7 @@ void queryMissing(StoreAPI & store, const PathSet & targets,
     PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
     unsigned long long & downloadSize, unsigned long long & narSize);
 
-bool willBuildLocally(Derivation & drv);
+bool willBuildLocally(const Derivation & drv);
 
 
 }