Invalid arg replacement in sys.config.src with {} in defaults

`replace_os_vars` function leaves closing brace `}` if default value contains `{}`

```
bash$ replace_os_vars <(echo '${A:-{1,2,3}}')
{1,2,3}
bash$ A=1 replace_os_vars <(echo '${A:-{1,2,3}}')
1}
bash$ A={5,6} replace_os_vars <(echo '${A:-{1,2,3}}')
{5,6}}
```