do not fail on first build failure when using --auto-build

When using reprotest with the --auto-build option, if a particular variations fails to build successfully, the build fails and reprotest stops. Ideally, further variations should continue to be tested, as some variations may trigger build failures.

If the "control" or "experiment-0" (e.g. no variations) builds fail, the build should probably just fail. If "experiment-1" (all variations) fails, the build should continue to test the individual variations (e.g. build_path, time, locale).