helpers (bash): modify create_tmp_file() to store files in self-contained dir

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

@@ -6,7 +6,10 @@ function container_exec() {
 
 function create_tmp_file() {
     name="$1"
-    tmp_path="/tmp/$@_$(date +%s)_$name"
+    prefix_dir="/tmp/zio"
+    tmp_path="$prefix_dir/$(date +%s).$name"
+
+    mkdir -p "$prefix_dir"
     touch "$tmp_path"
     echo "$tmp_path"
 }