iwyu.yml: fixed macOS run

[firewave]

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[firewave]

I assume this is also related to the wrong compiler being chosen in current runners. I hope it doesn't deteriorate even more over time...