nixos/mysqlBackup: set service Type

fixes #158802

Sets the mysql backup systemd service type to "oneshot" to ensure the
service is marked as started after the backup script fully proceeds. This
allows to reliably depend on completing of this service by other services.

nixos/modules/services/backup/mysql-backup.nix

@@ -113,9 +113,10 @@ in
         };
       };
       services.mysql-backup = {
-        description = "Mysql backup service";
+        description = "MySQL backup service";
         enable = true;
         serviceConfig = {
+          Type = "oneshot";
           User = cfg.user;
         };
         script = backupScript;