Can't restore when the config file name is custom

Can't restore when the config file name is custom:

```
root@perf ~/repl/1 # manticore-backup --backup-dir=/root/repl/1/backup --restore=backup-20230328051059
Copyright (c) 2023, Manticore Software LTD (https://manticoresearch.com)

Manticore config file:
Backup dir: /root/repl/1/backup
2023-03-28 05:21:33 [Info] Starting to restore...
2023-03-28 05:21:33 [Error] Failed to find config file in original backup
```

The config name is `manticore1.conf`:
```
root@perf ~/repl/1 # ls -la /root/repl/1/backup/backup-20230328051059/config/root/repl/1/
total 16
drwxr-xr-x 3 root root 4096 Mar 28 07:10 .
drwxr-xr-x 3 root root 4096 Mar 28 07:10 ..
drwxr-xr-x 2 root root 4096 Mar 28 07:10 data
-rw-r--r-- 1 root root 236 Mar 28 07:10 manticore1.conf
```

Reproduce the failure with the shown manticore-backup restore command and inspect how restore locates the original config file. Done means a backup containing the custom manticore1.conf file restores successfully instead of reporting that the config file cannot be found.

Written by the indexing model from the issue text.