CI improvements

[johnbillion]

This makes a few changes that improve the CI. They're a combination of various improvements I've made in my plugins.

• Splits up the CI into multiple workflows that run in parallel, reducing the overall run time from ~3 minutes to ~1 minute
• Switches to a specific WP version in Composer for better control over version testing, also updates from 5.6 to 5.7
• Dedicated workflow for testing against WordPress nightly once a week
• Standardises the cache key names

Before

After

[shadyvb]

Love it!