Builds: fix retry

[stsewd]

autoretry_for is broken,
using plain self.retry with a custom exception
works.

[humitos]

I haven't test this change, but I think a good way to test it in production could be to change the code in one instance and route builds from test-builds to that particular instance to make sure this works.

Besides, if we are not going to use autoretry_for, we should explain why we are not using it anymore in a code comment to avoid hitting the same problem. The documentation says that autoretry_for is the "syntax sugar way" when using Celery handlers. I missed the part of that comment in the review.