fix(copybuffer): prevent `which` alias usage (#12379)

plugins/copybuffer/copybuffer.plugin.zsh

@@ -2,7 +2,7 @@
 # onto the system clipboard
 
 copybuffer () {
-  if which clipcopy &>/dev/null; then
+  if builtin which clipcopy &>/dev/null; then
     printf "%s" "$BUFFER" | clipcopy
   else
     zle -M "clipcopy not found. Please make sure you have Oh My Zsh installed correctly."