Use libc++ instead of libstdc++11

Instead of using the linuxbrew one, install it via `conan install --requires='mold/[*]'`. 

```
ERROR: There are invalid packages:
mold/2.40.1: Cannot build for this configuration: Mold can only be built with libstdc++11; specify mold:compiler.libcxx=libstdc++11 in your build profile
```

- [ ] Conan mold