Tweak the way ccache is used during CI

Tweak the way ccache is used during CI

Our current CI configuration causes ccache data to be zipped after each
job and also included in build artifacts, which will quickly become
infeasible as ccache data grows.  Instead of asking gitlab-runner to
preserve ccache data between jobs, keep a separate ccache directory on
each runner, expecting it to be accessible at /ccache when a CI job is
run.  As this requires gitlab-runner to be configured in a specific way,
do not use ccache at all in case the ccache directory is not found while
building.