Stop error "permission denied: /npm_completion" (#6340)

Fixes #5874

Related:
https://github.com/lukechilds/zsh-nvm/issues/23
https://github.com/zsh-users/antigen/issues/586

plugins/npm/npm.plugin.zsh

 (( $+commands[npm] )) && {
-    __NPM_COMPLETION_FILE="${ZSH_CACHE_DIR}/npm_completion"
+    __NPM_COMPLETION_FILE="${ZSH_CACHE_DIR:-$ZSH/cache}/npm_completion"
 
     if [[ ! -f $__NPM_COMPLETION_FILE ]]; then
         npm completion >! $__NPM_COMPLETION_FILE 2>/dev/null