container-volume-backup: Use symlink to latest file

Pointers to the latest backup are useful for restore operations, but
should be ignored in all other cases, and thus are more appropriately
handled as symbolic links.

host/base/systemd/container-volume-backup@.service

@@ -13,7 +13,7 @@ ExecStart=/bin/podman run --replace --rm --name %p-%i --entrypoint /bin/bash \
                            test "$(find /data -type f -newer /backup/%i-latest.tar.gz -print -quit | wc -l)" -eq 0 && exit 0; \
                            name="%i-$(date +%%w%%H)" && \
                            tar --verbose --create --gzip --file "/backup/$name.tar.gz" --directory /data . && \
-                           ln --force "/backup/$name.tar.gz" /backup/%i-latest.tar.gz'
+                           cd /backup && ln --symbolic --force "$name.tar.gz" %i-latest.tar.gz'
 
 [Install]
 WantedBy=multi-user.target