gitlab-ci: use ccache to speed up builds
The build currently says:
No URL provided, cache will be not downloaded from shared cache server. Instead a local version of cache will be extracted.
This is because distributed runners caching isn't enabled on FD.o yet, but it might be in the future.
Without this, the cache is local to each runner, which means fewer cache hits, but it's still a big help overall.
Edited by Eric Engestrom