unable to open config file
**Describe the bug**
When running `xmrig --config=config.json` I receive the error that it was unable to open the config file. Currently I'm trying to run it as a systemd service.

**To Reproduce**
1. Create a `config.json` file with the following content `{ "url": "127.0.0.1:3333" }`
2. Run `xmrig --config=config.json`

**Expected behavior**
A clear and concise description of what you expected to happen.

**Required data**
 - Miner log as text or screenshot
 ```
victor@t495:/tmp/xmrig$ ls
config.json
victor@t495:/tmp/xmrig$ cat config.json 
{
  "url": "127.0.0.1:3333"
}
victor@t495:/tmp/xmrig$ xmrig --config=config.json
[2021-11-26 14:03:02.529] unable to open "/nix/store/zayvp5khqxlhyfjdz0dwn47fi0v70cvc-xmrig-6.15.0/bin/config.json".
[2021-11-26 14:03:02.529] unable to open "/home/victor/.xmrig.json".
[2021-11-26 14:03:02.529] unable to open "/home/victor/.config/xmrig.json".
[2021-11-26 14:03:02.529] no valid configuration found, try https://xmrig.com/wizard
```
 - Config file or command line (without wallets)
```json
{
  "url": "127.0.0.1:3333"
}
```
 - OS: NixOS


