desi_compute_bias requires unsetting $DESI_SPECTRO_DARK

[sbailey]

To create a new bias with desi_compute_bias, you first have to unset DESI_SPECTRO_DARK, or otherwise it fails due to not finding the bias file that you are trying to create in the first place.

I did not find this procedure documented, but I have a vague memory of @julienguy mentioning needing to do this. We should fix this so that "missing" biases and darks are only fatal if the processing is actually needs that bias/dark.