Bug 31496: Fix branch in printslip.pl to logged in staff user's library

Bug 31496: Fix branch in printslip.pl to logged in staff user's library

When you print a receipt slip in SCO, the library/branch you are at
in should be fetched from the logged in staff member's data (staff login
done before entering SCO view) instead of the patron's own who has signed
in to the SCO. Printing a slip shows patron's own home branch in the slip
as the branch they're in. This causes confusion with patrons who want to
print receipts/slips.

Test plan:
1) Make sure the staff user's library (used to login into SCO patron
   view) is not the same as the patron's who is logging into SCO
2) Print a slip in SCO and notice patron's home library printed on the slip
3) Apply patch
4) Print a slip in SCO and notice the library is now the staff user's library
   used to sign in to SCO patron view.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>