Prefers bash over sh.

Assuming a bash interactive environment will be correctly formed on login, we
should prefer to probe the environment using a shell that reports itself as
`bash` instead of `sh` which may not source files that set the
environment modules in statements like:

```
case "$is" in
bash) test -s /etc/bash.bashrc.local && . /etc/bash.bashrc.local ;;
ksh)  test -s /etc/ksh.kshrc.local   && . /etc/ksh.kshrc.local ;;
zsh)  test -s /etc/zsh.zshrc.local   && . /etc/zsh.zshrc.local ;;
ash)  test -s /etc/ash.ashrc.local   && . /etc/ash.ashrc.local
esac
test -s /etc/sh.shrc.local && . /etc/sh.shrc.local
```

lib/spack/spack/platforms/cray.py

@@ -27,7 +27,7 @@ def _target_from_clean_env(name):
         # CAUTION - $USER is generally needed to initialize the environment.
         # There may be other variables needed for general success.
         output = env('USER=%s' % os.environ['USER'],
-                    '/bin/sh', '--noprofile', '-c',
+                    '/bin/bash', '--noprofile', '--norc', '-c',
                     '. /etc/profile; module list -lt',
                     output=str, error=str)
         default_modules = [i for i in output.splitlines()