add `spack --print-shell-vars` to speed up `setup-env.[c]sh` (#8101)

- The setup-env.sh script currently makes two calls to spack, but it
  should only need to make one.

- Add a fast-path shell setup routine in `main.py` to allow the shell
  setup to happen in a single, fast call that doesn't load more than it
  needs to.

- This simplifies setup code, as it has to eval what Spack prints

- TODO: consider eventually making the whole setup script the output of a
  spack command