With 15 runners we get 150GB of cache space to use, and we currently
have 18 jobs using ccache.
Although each run only generates ~ 200-300 MB of cache data on each run,
the small the small size may be contributing to poor hitrate.
Bump ccache to 2GB per job ~ --> 36GB of the total 150GB cache space to
try and reduce any thrashaing and generally increase hitrate.
580e9eefe3