helpers (bash): fix overridable prefix dir not containing "zio" folder

usr/local/libexec/zio/helpers/bash.sh

@@ -11,10 +11,10 @@ function die() {
 
 function get_config_dir() {
     prog="$(echo $1 | sed -e "s/sh.zio.//g")"
-    prefix="/etc/zio"
+    prefix_dir="/etc/zio"
 
     if [[ -n "$2" ]]; then
-        prefix="$2"
+        prefix_dir="$2/zio"
     fi
 
     config_dir=""