Return more info from BinaryCacheStore::addToStoreCommon

We don't need it yet, but we could/should in the future, and it's a
cost-free change since we already have the reference. I like it.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/binary-cache-store.hh

@@ -72,7 +72,7 @@ private:
 
     void writeNarInfo(ref<NarInfo> narInfo);
 
-    StorePath addToStoreCommon(
+    ref<const ValidPathInfo> addToStoreCommon(
         Source & narSource, RepairFlag repair, CheckSigsFlag checkSigs,
         std::function<ValidPathInfo(HashResult)> mkInfo);