add logic to try to run nodejs first then node second to fix ubuntu

ubuntu use "nodejs" as the Node.js executable if you do "sudo apt-get install nodejs". you can do "sudo apt-get install nodejs-legacy" to get both "nodejs" and "node". But not everybody knows this.

Today our CLI assume you have "node". This has caused quite a few issues with customers who complains about the installation experience.

The fix here is to try to find "nodejs" first
1. If I can find it, then it runs "nodejs"
2. Otherwise, it runs "node"
change to `if [ -n "$node_exec" ]`
or leave as -z, remove the ! and switch the order.