Job startup find-ifdh code should have a fallback from "current"...

[marcmengel]

We should probably have the code in the simple.sh template in setup_ifdh_env() that looks like:

    . /cvmfs/fermilab.opensciencegrid.org/packages/common/setup-env.sh
    sos=$(spack arch --operating-system)
    spack env activate ifdh_env_${sos}_${IFDH_VERSION:-current} ||
      echo Falling back to current ifdhc for this operation >&2 &&
      spack env activate ifdh_env_${sos}_current

have a second fall-back to ifdh_env_${sos}_prev (i.e. the previous known working version), in case rolling out
the new ifdh version goes wonky (as it did yesterday) Then we would get a notice that the new environment is
borked, but peoples jobs would hopefully still run and copy back output with the previous version. We will of course
have to have those _prev environment symlinks defined for this to work.