use rsync's --chmod instead of "find [...] -exec chmod [...]" in mirror-2stage.sh

mirror-2stage.sh does post-sync fixups as follows:

files/mirror-2stage.sh:find ${DEST_DIR} -type d ! -perm /o=r,o=x -ls -exec chmod go+rx '{}' +
files/mirror-2stage.sh:find ${DEST_DIR} -type f ! -perm /o=r -ls -exec chmod go+r '{}' +

First of all, this is racy.

Second of all, it's not clear where this came from and if it's still needed.

Third of all, rsync's --chmod option can be used to alter its
view of the source's permissions such that we can use it to
replace the find commands above.

Let us do so.