ci: bump CCACHE_MAXSIZE to 2G

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.

ci/test/00_setup_env.sh

@@ -48,7 +48,7 @@ export RUN_FUZZ_TESTS=${RUN_FUZZ_TESTS:-false}
 export BOOST_TEST_RANDOM=${BOOST_TEST_RANDOM:-1}
 # See man 7 debconf
 export DEBIAN_FRONTEND=noninteractive
-export CCACHE_MAXSIZE=${CCACHE_MAXSIZE:-500M}
+export CCACHE_MAXSIZE=${CCACHE_MAXSIZE:-2G}
 export CCACHE_TEMPDIR=${CCACHE_TEMPDIR:-/tmp/.ccache-temp}
 export CCACHE_COMPRESS=${CCACHE_COMPRESS:-1}
 # The cache dir.