Fix spack-stack path env for Hercules

The `MODULEPATH` for Hercules is currently pointing to the `unified-env` env under `spack-stack/1.6.0` but it should be pointing to the `gsi-addon-env` env. See https://github.com/NOAA-EMC/Fit2Obs/blob/develop/modulefiles/fit2obs_hercules.lua#L5

@WalterKolczynski-NOAA encountered issues while trying to build the new `wflow.1.1.4` tag on Hercules.