better performance in find_firsts

_is_build_cache not set in dictionary storage

tiramisu/storage/dictionary/option.py

@@ -399,6 +399,7 @@ class StorageOptionDescription(StorageBase):
                 _currpath.pop()
         if save:
             self._cache_paths = (tuple(cache_option), tuple(cache_path))
+            self._is_build_cache = True
 
     def impl_get_options_paths(self, bytype, byname, _subpath, only_first, context):
         find_results = []