Incremental prepare fails when innodb_undo_tablespaces > 0

In windows, Mariabackup incremental prepare fails to open undo tablespaces while applying
the delta file. It leads to failure of incremental prepare.

Scenario:
========

perl mysql-test-run.pl mariabackup.incremental_encrypted --mysqld=--innodb_undo_tablespaces=0

Problem is that backup opens the undo tablespaces in srv_undo_tablespaces_init() and it tries
to open again while applying the delta, but later is in conflicting mode and it leads to
the failure. we found out while testing MDEV-29986. Thanks to wlad for debugging the windows failure.