codesign: fix script to support spaces in the CODESIGN_IDENTITY

Change-Id: Ib30717518ff1d1291ba3a1fc7ba045e1fa29ad88
Reviewed-on: https://gerrit.libreoffice.org/6594
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>