OC Sponsors: GIFTCARD users are duplicated added

[beta-ziliani]

It appears that users with GIFTCARD payment method get added to a fictitious "Open Collective" account (#434 (comment)).

The problematic record in the CSV as exported by OC is (redacted):

"2023-04-01T03:15:56","Redacted","Redacted","Monthly contribution from Redacted (Coffee backer)","CREDIT","CONTRIBUTION","","","","$5.00 USD",5,-0.45,4.55,12477.9,"USD","crystal-lang","Crystal Language","Redacted","Redacted","OPENCOLLECTIVE","GIFTCARD",,"",,,