setup-env: Avoid different output format of ps (#9629)

setup-env includes a call to 'ps' to determine what shell is being
used. 'ps' can be instructed to use a different default output format
via the 'PS_FORMAT' env variable. Thus unset this variable before
calling 'ps'.

share/spack/setup-env.sh

@@ -199,7 +199,7 @@ export SPACK_ROOT=${_sp_prefix}
 # Determine which shell is being used
 #
 function _spack_determine_shell() {
-	ps -p $$ | tail -n 1 | awk '{print $4}' | sed 's/^-//' | xargs basename
+	PS_FORMAT= ps -p $$ | tail -n 1 | awk '{print $4}' | sed 's/^-//' | xargs basename
 }
 export SPACK_SHELL=$(_spack_determine_shell)