fix fallback to DESI_SPECTRO_CALIB if DESI_SPECTRO_DARK is set and no suitable files found

[Waelthus]

This fixes #1880 by falling back to DESI_SPECTRO_CALIB in cases where no suitable darks are found in DESI_SPECTRO_DARK.

[sbailey]

Thanks for the quick fix, I'll merge this now so that we can proceed, but in the near term let's get DESI_SPECTRO_DARK completely self-sufficient for DARKs covering the full time range, even if that means copying darks from DESI_SPECTRO_CALIB into DESI_SPECTRO_DARK to cover the earlier time period. i.e. we should aim to deprecate all biases and darks in DESI_SPECTRO_CALIB so that DESI_SPECTRO_DARK is the complete and authoritative source of those.