Add a ca-derivations required machine feature

Make ca-derivations require a `ca-derivations` machine feature, and
ca-aware builders expose it.

That way, a network of builders can mix ca-aware and non-ca-aware
machines, and the scheduler will send them in the right place.

src/libstore/store-api.hh

@@ -180,6 +180,8 @@ struct StoreConfig : public Config
 
     StoreConfig() = delete;
 
+    StringSet getDefaultSystemFeatures();
+
     virtual ~StoreConfig() { }
 
     virtual const std::string name() = 0;
@@ -196,7 +198,7 @@ struct StoreConfig : public Config
 
     Setting<bool> wantMassQuery{this, false, "want-mass-query", "whether this substituter can be queried efficiently for path validity"};
 
-    Setting<StringSet> systemFeatures{this, settings.systemFeatures,
+    Setting<StringSet> systemFeatures{this, getDefaultSystemFeatures(),
         "system-features",
         "Optional features that the system this store builds on implements (like \"kvm\")."};