Use %B for Harbormaster build log updates as well

Summary: So I derped and missed the %s inside the UPDATE query (previously only fixing the INSERT query). This changes %s to %B for the update logic as well.

Test Plan: Patched it in production and saw the offending build run all the way through without UTF8-related exceptions.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11669