Remove the pacstrap directory early to lower the maximum size of the working directory
When building images containing large packages like CUDA or TensorFlow, it's easy to consume around 20-30 GB of temporary disk space (pacstrap directory + airootfs image + iso image). This patch minimizes disk usage by removing the pacstrap directory early (if the -r flag is enabled), making it easier to use a tmpfs for the working directory. I'm happy about feedback if this code snippet should move to another function.
