Bug 33048: Fix email link on error page when opac login not allowed

Bug 33048: Fix email link on error page when opac login not allowed

This change replaces a non-existent variable with the
KohaAdminEmailAddress syspref.

Test plan:
0. Apply the patch
1. Go to http://localhost:8081/cgi-bin/koha/admin/preferences.pl?op=search&searchfield=opacuserlogin
2. Set to "Don't allow"
3. In an incognito window, go to http://localhost:8080/cgi-bin/koha/opac-reportproblem.pl
4. Note that the "mailto" link for "Email" has the value from KohaAdminEmailAddress syspref

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>