version_handlers_builder: Use string references

Change-Id: Iefa4b09c2952eb5f6e7da4509c1a7a39dead9ed7
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/bmc/version-handler/version_handlers_builder.cpp b/bmc/version-handler/version_handlers_builder.cpp
index d386128..d6ed016 100644
--- a/bmc/version-handler/version_handlers_builder.cpp
+++ b/bmc/version-handler/version_handlers_builder.cpp
@@ -61,7 +61,7 @@
             /* version must have handler */
             const auto& h = v.at("handler");
 
-            const std::string handlerType = h.at("type");
+            const std::string& handlerType = h.at("type");
             if (handlerType == "file")
             {
                 const auto& path = h.at("path");
@@ -80,7 +80,7 @@
 
             /* to make an action optional, assign type "skip" */
             const auto& onOpen = a.at("open");
-            const std::string onOpenType = onOpen.at("type");
+            const std::string& onOpenType = onOpen.at("type");
             if (onOpenType == "systemd")
             {
                 pack->onOpen = std::move(buildSystemd(onOpen));