SqliteZipBackend: Return self in store

The `store` method of the `SqliteEntityOverride` class, used by the
`SqliteZipBackend` storage backend (and with that all other backends to
subclass this), did not return `self`. This is in conflict with the
signature of the base class that it is overriding.

Since the `SqliteZipBackend` is read-only and so `store` would never be
called, this problem went unnoticed. However, with the addition of the
`SqliteDosStorage` backend which is *not* read-only, this bug would
surface when trying to store a node since certain methods rely on this
method returning the node instance itself.

aiida/storage/sqlite_zip/orm.py

@@ -70,7 +70,7 @@ def store(self, *args, **kwargs):
         backend = self._model._backend  # pylint: disable=protected-access
         if getattr(backend, '_read_only', False):
             raise ReadOnlyError(f'Cannot store entity in read-only backend: {backend}')
-        super().store(*args, **kwargs)  # type: ignore # pylint: disable=no-member
+        return super().store(*args, **kwargs)  # type: ignore # pylint: disable=no-member
 
 
 class SqliteUser(SqliteEntityOverride, users.SqlaUser):