Fix virtual method calls during construction in S3BinaryCacheStoreImpl

Move init() call from constructor to openStore() method to avoid calling
virtual methods during object construction. This prevents undefined
behavior when virtual methods are called before the object is fully
constructed.

src/libstore/s3-binary-cache-store.cc

@@ -289,8 +289,6 @@ struct S3BinaryCacheStoreImpl : virtual S3BinaryCacheStore
         , s3Helper(config->profile, config->region, config->scheme, config->endpoint)
     {
         diskCache = getNarInfoDiskCache();
-
-        init();
     }
 
     std::string getUri() override
@@ -597,10 +595,12 @@ struct S3BinaryCacheStoreImpl : virtual S3BinaryCacheStore
 
 ref<Store> S3BinaryCacheStoreImpl::Config::openStore() const
 {
-    return make_ref<S3BinaryCacheStoreImpl>(ref{
+    auto store = make_ref<S3BinaryCacheStoreImpl>(ref{
         // FIXME we shouldn't actually need a mutable config
         std::const_pointer_cast<S3BinaryCacheStore::Config>(shared_from_this())
     });
+    store->init();
+    return store;
 }
 
 static RegisterStoreImplementation<S3BinaryCacheStoreImpl::Config> regS3BinaryCacheStore;