Fixed deserialization of new artifacts (and possibly some other objects)

Was broken in my previous PR, since pointer graph serialization was
enabled by default, leading to deserializationFix triggering on netpack
apply.

Cleaned up / clarified code

lib/serializer/CLoadFile.cpp

@@ -29,6 +29,7 @@ int CLoadFile::read(std::byte * data, unsigned size)
 
 void CLoadFile::openNextFile(const boost::filesystem::path & fname, ESerializationVersion minimalVersion)
 {
+	serializer.loadingGamestate = true;
 	assert(!serializer.reverseEndianness);
 	assert(minimalVersion <= ESerializationVersion::CURRENT);