multiple pivot tables on same sheet OOXML fix (part1), related #i83250#

This just fixes the generation of the pivot table file and the
relationship. It crashed in a dbgutil build because we tried to
overwrite the same stream for each pivot table on one sheet.

Change-Id: If2c9541e38b483ead75fff32d5f6d9e16970e702
Reviewed-on: https://gerrit.libreoffice.org/8097
Tested-by: Kohei Yoshida <libreoffice@kohei.us>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>