cray: add comment about module use in packages.yaml (#7751)
cray: add comment about module use in packages.yaml (#7751)

This advises users to prefer `modules:` entries in `packages.yaml` on Cray.
matthiasdiener/spack,tmerrick1/spack,LLNL/spack,mfherbst/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,EmreAtes/spack,tmerrick1/spack,matthiasdiener/spack,EmreAtes/spack,krafczyk/spack,krafczyk/spack,matthiasdiener/spack,krafczyk/spack,iulian787/spack,krafczyk/spack,mfherbst/spack,tmerrick1/spack,tmerrick1/spack,LLNL/spack,matthiasdiener/spack,LLNL/spack,EmreAtes/spack,mfherbst/spack,EmreAtes/spack,mfherbst/spack,tmerrick1/spack,mfherbst/spack,krafczyk/spack,matthiasdiener/spack,iulian787/spack,EmreAtes/spack