colorize: add compatibility for zsh < 5.1 (#9400)

The direct assignment of a local array is not possible in older ZSH version, as used by RHEL/CentOS 7.

plugins/colorize/colorize.plugin.zsh

@@ -6,7 +6,8 @@ alias cless="colorize_less"
 ZSH_COLORIZE_PLUGIN_PATH=$0:A
 
 colorize_check_requirements() {
-    local available_tools=("chroma" "pygmentize")
+    local -a available_tools
+    available_tools=("chroma" "pygmentize")
 
     if [ -z "$ZSH_COLORIZE_TOOL" ]; then
         if (( $+commands[pygmentize] )); then