gha: use ccache and cache action

[dtrifiro]

In #145 we started using the cache action to speed up builds by caching the python virtual environments in .nox.

Even though the CPU build is quite fast, we could speed this up even further by installing ccache and caching /root/.cache/ccache before actually building/installing vllm.

[dtrifiro]

This was (partially) implemented in #147.

The issue with this implementation (and in #145 's) as well, is that all three workflows use the same cache key, meaning there is contention between the three jobs for the cache key.