Add caching to the phpcs pipeline job

Problem/Motivation

This is the only linting step that doesn't have cache implemented, but phpcs supports it - so hopefully we can do a similar thing to other lint jobs.

The current CPU request then could be reduced to 4 or so from 16 once it's working.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet