Remove existing apk cache symlink before creating link
This removes any existing symlinks (which always seem to be broken when this is encountered) to /chroot_native/etc/apk/cache before creating the symlink.
Without this, I see this error fairly frequently when building:
[17:47:26] ERROR: Command failed: % sudo ln -s /var/cache/apk /home/clayton/src/pmbootstrap-out/chroot_native/etc/apk/cache