On Cray platforms, we rely heavily on the module system to figure out
what targets, compilers, etc. are available. This unfortunately means
that we shell out to the `module` command as part of platform
initialization.

Because we run subcommands in a shell, we can get infinite recursion if
`setup-env.sh` and friends are in some init script like `.bashrc`.

This fixes the infinite loop by adding guards around `setup-env.sh`,
`setup-env.csh`, and `setup-env.fish`, to prevent recursive
initializations of Spack. This is safe because Spack never shells out to
itself, so we do not need it to be initialized in subshells.

- [x] add recursion guard around `setup-env.sh`
- [x] add recursion guard around `setup-env.csh`
- [x] add recursion guard around `setup-env.fish`