fix llvm_out to find 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>

src/bootstrap/src/lib.rs

@@ -801,7 +801,8 @@ impl Build {
     /// Note that if LLVM is configured externally then the directory returned
     /// will likely be empty.
     fn llvm_out(&self, target: TargetSelection) -> PathBuf {
-        self.out.join(&*target.triple).join("llvm")
+        let dir = if self.config.llvm_from_ci { "ci-llvm" } else { "llvm" };
+        self.out.join(&*target.triple).join(dir)
     }
 
     fn lld_out(&self, target: TargetSelection) -> PathBuf {