Resolves T5987. This build step was at some point converted to use yielding, which meant that whenever the build step executes it will create a new log. This checks to see if there is an existing log before creating a new one and uses that instead.
Long term we're going to need some way of attaching data to PhabricatorWorkerYieldException that can be read when the build step starts again; this will allow us to move more build steps off while (...) { ... sleep(X); } loops and onto yielding.