fix prebuilt_llvm_config to use the correct LLVM root

When `download-ci-llvm` is enabled, `prebuilt_llvm_config` 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/core/build_steps/llvm.rs

@@ -97,7 +97,12 @@ pub fn prebuilt_llvm_config(
     let root = "src/llvm-project/llvm";
     let out_dir = builder.llvm_out(target);
 
-    let mut llvm_config_ret_dir = builder.llvm_out(builder.config.build);
+    let mut llvm_config_ret_dir = if builder.config.llvm_from_ci {
+        builder.config.ci_llvm_root()
+    } else {
+        builder.llvm_out(builder.config.build)
+    };
+
     if !builder.config.build.is_msvc() || builder.ninja() {
         llvm_config_ret_dir.push("build");
     }