We don't need a read-write key: we can just SSH directly to
jim-backups@backup.jim.sh instead and run commands thay way.
Remove read-write key and document it in the README.

Also add some tools to update the README variables on updates.

This prevents it from e.g. needing a specific glibc version on the
client.

Put our own binary in here, so we can keep it updated with local
patches more easily.  Also add build instructions.
This one is built from
https://github.com/borgbackup/borg/pull/6011