CI: Rebuild WordPress every day

[adamziel]

Playground uses a bundled version of WordPress, which means it needs to periodically update it to keep up with new releases (especially the Beta releases).

Building a bundle involves running a command with a specific version:

nx compile-wordpress playground-remote --preset=5.9

Let's:

• Add a blanket command npm run compile-wordpress
• Run it using a scheduled GitHub Action
• Commit the result to the repo

Ideally the action would short-circuit if the latest WordPress version hasn't changed since the last run.

Then, once playground.wordpress.net can be automatically deployed, let's deploy it whenever a new bundle is committed.

[adamziel]

Shipped in #572