ci: Bump default CCACHE_MAXSIZE to 500M

This also allows to drop individually hardcoded values, which are
impossible to overwrite and hard to maintain.

ci/test/00_setup_env_native_asan.sh

@@ -32,4 +32,3 @@ export BITCOIN_CONFIG="\
  -DAPPEND_CXXFLAGS='-std=c++23' \
  -DAPPEND_CPPFLAGS='-DARENA_DEBUG -DDEBUG_LOCKORDER' \
 "
-export CCACHE_MAXSIZE=300M