Add caching for Linux dependencies

**What is the current behaviour and why should it be changed?**

Currently, dependencies for the build process not part of the base Github runner are retrieved as part of the CI runs.

On the day planned for the 3.12.0 release, the CI processes failed because ppa.launchpad.net was suffering a cyberattack and not able to respond to requests.

**Describe possible approaches**

We already cache certain dependencies. I don't know the details of the problem well enough to suggest potential fixes.

However, any solution should still check the remote _first_ and only fall back to the cache when the remote isn't available.

**Has this feature been discussed and generally agreed?**

Not yet, although caching is done elsewhere - but more for speed than resilience.

No specific files, tests, or entry points are named. Inspect the CI configuration and the existing dependency-cache setup first, then determine how to check the remote before using cached Linux dependencies; done means CI can fall back to the cache when the remote is unavailable.

Written by the indexing model from the issue text.