setup-env.sh: send cd output to /dev/null when it affects Spack

- We've seen this a few times now where users have set up `cd` to echo
  the new directory, and it screws up `setup-env.sh`

- In the past we've said this is user error.

- Here, we just fix it by sending `cd` output to /dev/null where needed.
  - this works in bash, zsh, and dash