brew: fix disclaimer shown if mkdir is aliased

Fixes #5623.

plugins/brew/brew.plugin.zsh

@@ -3,7 +3,7 @@ alias bubo='brew update && brew outdated'
 alias bubc='brew upgrade && brew cleanup'
 alias bubu='bubo && bubc'
 
-if mkdir "$ZSH_CACHE_DIR/.brew-completion-message" 2>/dev/null; then
+if command mkdir "$ZSH_CACHE_DIR/.brew-completion-message" 2>/dev/null; then
 	print -P '%F{yellow}'Oh My Zsh brew plugin:
 	cat <<-'EOF'