[buildbot] Fix getIgnoreList dangling arg

Summary:
Missed during cleanup of D7787 that wasn't caught by tests
because this method was mocked for all tests that called it.

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7824