Fix handling of MODULEPATH in setup-env.sh (#11274)

Usage of double quotes was preventing word-splitting when parsing
module roots in setup-env.sh, which lead to an error when multiple
module roots are used (in particular when Spack is pointed to use
an upstream module root in addition to its own).

share/spack/setup-env.sh

@@ -295,7 +295,7 @@ _sp_multi_pathadd() {
     if  [[ -n "${ZSH_VERSION:-}" ]]; then
         setopt sh_word_split
     fi
-    for pth in "$2"; do
+    for pth in $2; do
         _spack_pathadd "$1" "${pth}/${_sp_sys_type}"
     done
 }