make custom prompts more compatible with p9k

internal/p10k.zsh

@@ -1141,9 +1141,10 @@ prompt_host() {
 _p9k_custom_prompt() {
   local segment_name=${1:u}
   local command=POWERLEVEL9K_CUSTOM_${segment_name}
-  local -a cmd=("${(@Q)${(z)${(P)command}}}")
-  whence $cmd[1] &>/dev/null || return
-  local content="$("$cmd[@]")"
+  command=${(P)command}
+  local cmd="${(Q)${(z)command}[1]}"
+  (( $+functions[$cmd] || $+commands[$cmd] )) || return
+  local content="$(eval $command)"
   [[ -n $content ]] || return
   _p9k_prompt_segment "prompt_custom_$segment_name" $_p9k_color2 $_p9k_color1 "CUSTOM_${segment_name}_ICON" 0 '' "$content"
 }