fix `llvm_out` to use the correct LLVM root

When `download-ci-llvm` is enabled, `llvm_out` ends up with the
error below due to an incorrect path on cross-compilations. This change fixes that.

```
failed to execute command: "/rust/build/x86_64-unknown-linux-gnu/llvm/build/bin/llvm-config" "--version"
ERROR: No such file or directory (os error 2)
```

Signed-off-by: onur-ozkan <work@onurozkan.dev>