gitlab/cerbero: Don't fail when there's no macOS cache
test -d "${CERBERO_HOME}/dist/${ARCH}"
will cause the build to error
out if the dist dir doesn't exist, which will happen if there's no
cache, for example:
https://gitlab.freedesktop.org/seungha.yang/gst-plugins-bad/-/jobs/7405565
What we actually want to do, is to not perform the rsync if there's no cache, since it means we want to rebuild from scratch; for instance, if the current MR uses a custom cerbero branch of the same name which won't have a prebuilt cache.