use read-only mount-point

[moustakas]

NERSC recommends using the read-only mount-point for CFS, /dvs_ro/cfs--
https://docs.nersc.gov/performance/io/dvs/

The changes in fastspecfit.io can probably be modeled on desihub/desispec#1856.

[moustakas]

Done in https://github.com/desihub/fastspecfit/tree/9d15d5fec41f66a78b89c02035ae567daadba16f.