Save fully interpolated run config when starting a new run

```
from omegaconf import OmegaConf

@hydra.main(version_base="1.3", config_path="config", config_name="config")
def main(cfg: DictConfig) -> None:                                                          
    print(OmegaConf.to_yaml(cfg, resolve=True))                                             
    OmegaConf.save(cfg, "run-config.yaml")

main()
```