chore: Add npm update to build script

[tofumatt]

Since #6629, npm 6 in the minimum version, so install it at setup time. Really this installs the latest version because I didn't want to deal with adding yet another 6 somewhere. If that's too dangerous we can change this to be explicitly 6–let me know.

This threw me because I created a PR and ran the app with npm 5 and received no error messages. It wasn't until my noisy PR came in that @gziolo pointed out I was running an out-of-date npm.

Description

Globally install npm during set up.

How has this been tested?

Ran the script on a new machine; it worked.

Types of changes

Add npm install npm -g to start up.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[gziolo]

Good idea 👍