Use %B when writing to Harbormaster build logs

Summary: Fixes T7007. Using %B permits non-UTF8 data to be appended to Harbormaster build logs. Since we're not really in control of the processes Harbormaster is running remotely, and since they may output invalid UTF8 data, we should store the invalid data instead of failing the build (due to UTF8 exception).

Test Plan: @epriestley said this was the right fix, though I haven't tested it on our production system which actually exhibits the issue yet.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7007

Differential Revision: https://secure.phabricator.com/D11532