CI runtime: add ccache

[padelsbach]

Enable ccache when building in CI.

The CI jobs all rebuild the latest wolfSSL tip each run. Most significantly, MacOS build is slow, and caching drops the build from around 42s to about 12s -- and at 22 jobs, this saves about 10-11 minutes of runner time for Mac. But note that Mac is currently the long-pole in CI and has the smallest runner pool.

Ubuntu build is approx 12s today, so the savings is less there, just a few total minutes across all jobs. Examples are excluded due to small runtimes and they'd cause excessive writes to the cache.

This addition closely follows the way ccache is used in wolfSSL:

• PRs don't update the cache, only runs from the main fork update it
• uses a "fallback to latest cache entry" approach
• scheduled/cron run to refresh the cache weekly

At a later point, we may want to pin the version of wolfSSL used in wolfHSM CI for more determinism and speed.