mirrorer: move logfile before flat site directory
mirrorer: move logfile before flat site directory

If the flat site directory doesn't exist, the move command will fail,
and we'll lose the logfile which might have explained why the flat site
directory didn't exist in the first place.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet