DESI_SPECTRO_SIM contains simulated raw data, with the canonical location
$DESI_ROOT/spectro/sim. Simulated files are additionally grouped under
a $PIXPROD subdirectory to isolate different simulation runs.
The intention is that $DESI_SPECTRO_SIM/$PIXPROD will contain a superset of the files in $DESI_SPECTRO_DATA, such that one could set DESI_SPECTRO_DATA=$DESI_SPECTRO_SIM/$PIXPROD and run the pipeline. The simulator does not yet output truly raw data (only preprocessed data) so this is not yet possible.