ci: Fix macOS-cross SDK rsync

This should fix the macOS-cross build on Cirrus CI containers.

Locally this was already working, because the SDK was cached in
/ci_container_base/ in the image, which is also the folder used for a
later CI run.

However, on Cirrus CI, when using an image *and* a custom BASE_ROOT_DIR,
the SDK will not be found in /ci_base_install/, nor in BASE_ROOT_DIR.

Fix this by normalizing *all* folders to /ci_container_base/.

ci/test/00_setup_env.sh

@@ -13,13 +13,9 @@ set -ex
 BASE_READ_ONLY_DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )"/../../ >/dev/null 2>&1 && pwd )
 export BASE_READ_ONLY_DIR
 # The destination root dir inside the container.
-if [ -z "${DANGER_RUN_CI_ON_HOST}" ] ; then
-  # This folder only exists on the ci guest and will be a copy of BASE_READ_ONLY_DIR
-  export BASE_ROOT_DIR="/ci_container_base"
-else
-  # This folder is equal to BASE_READ_ONLY_DIR and is read-write
-  export BASE_ROOT_DIR="${BASE_READ_ONLY_DIR}"
-fi
+# This folder will also hold any SDKs.
+# This folder only exists on the ci guest and will be a copy of BASE_READ_ONLY_DIR
+export BASE_ROOT_DIR="/ci_container_base"
 # The depends dir.
 # This folder exists only on the ci guest, and on the ci host as a volume.
 export DEPENDS_DIR=${DEPENDS_DIR:-$BASE_ROOT_DIR/depends}