Backup: Borg: All logs as JSON please

That makes working with its output in Grafana easier.
Good documentation can be found at
https://borgbackup.readthedocs.io/en/stable/internals/frontends.html#json-output

morph/lib/borgbackup.nix

@@ -58,6 +58,8 @@ in {
 
 	# Output statistics after uploading a backup set
 	extraCreateArgs = "--stats --json";
+	# All logs in JSON to help Prometheus/Grafana
+	extraArgs = "--log-json";
 
         # Ciphertext doesn't compress well
         compression = "none";