Use ccache to speed up build times
We can speed up the CI by installing ccache on all of the GitLab CI runners, and using GitLab CI caching to share the $CCACHE_DIR between jobs.
For the Linux runners we can install the ccache package inside the containers. For OSX and Windows the admin of those machines will need to install it in the host.
It's also important that the GitLab CI machines have a reasonable about of disk space dedicated to the CI cache -- at least large enough to fit all the .o files from a build of PD.