Factor out Machine::systemSupported

There's just enough logic (the `"builtin"` special case) that makes this
worthy of its own method.

src/libstore/machines.hh

@@ -19,6 +19,12 @@ struct Machine {
     const std::string sshPublicHostKey;
     bool enabled = true;
 
+    /**
+     * @return Whether `system` is either `"builtin"` or in
+     * `systemTypes`.
+     */
+    bool systemSupported(const std::string & system) const;
+
     /**
      * @return Whether `features` is a subset of the union of `supportedFeatures` and
      * `mandatoryFeatures`