In autobuilder `--debug`, drop into debug shell if products are unsatisfied

[Keno]

Usually in that case something went wrong, so it would be useful to have a shell to look around at what actually happened.

[giordano]

Good idea, but to be clear I don't think we can recreate exactly the same environment as before, for example environment variables set before would have been gone, like anything that had been written to tmpfs