cloudapp: fix permissions of credentials file

Fixes #4151

plugins/cloudapp/cloudapp.plugin.zsh

-#!/bin/zsh
-alias cloudapp=$ZSH/plugins/cloudapp/cloudapp.rb
+alias cloudapp="${0:a:h}/cloudapp.rb"
+
+# Ensure only the owner can access the credentials file
+if [[ -f ~/.cloudapp ]]; then
+  chmod 600 ~/.cloudapp
+fi