make __p9k_dump_file readonly to ensure no one unsets it

powerlevel9k.zsh-theme

@@ -17,8 +17,7 @@
 [[ ! -o 'no_brace_expand' ]] || __p9k_src_opts+=('no_brace_expand')
 'builtin' 'setopt' 'no_aliases' 'no_sh_glob' 'brace_expand'
 
-typeset -g __p9k_root_dir=${POWERLEVEL9K_INSTALLATION_DIR:-${${(%):-%x}:A:h}}
-typeset -g __p9k_dump_file=${XDG_CACHE_HOME:-~/.cache}/p10k-dump-${(%):-%n}.zsh
+(( $+__p9k_root_dir )) || typeset -gr __p9k_root_dir=${POWERLEVEL9K_INSTALLATION_DIR:-${${(%):-%x}:A:h}}
 
 () {
   emulate -L zsh
@@ -27,6 +26,7 @@ typeset -g __p9k_dump_file=${XDG_CACHE_HOME:-~/.cache}/p10k-dump-${(%):-%n}.zsh
     prompt_powerlevel9k_setup
     return
   fi
+  typeset -gr __p9k_dump_file=${XDG_CACHE_HOME:-~/.cache}/p10k-dump-${(%):-%n}.zsh
   if [[ -z $__p9k_dump_file(.zwc|)(#qNW) ]] && source $__p9k_dump_file 2>/dev/null && (( $+functions[_p9k_preinit] )); then
     _p9k_preinit
   fi