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/.
fa193f5dfc