Rollup merge of #124285 - ferrocene:unstable-L-rust-builtin, r=petrochenkov

Mark `@RUSTC_BUILTIN` search path usage as unstable

Follow up to https://github.com/rust-lang/rust/pull/121843

r? `@petrochenkov`

compiler/rustc_interface/src/tests.rs

@@ -321,6 +321,7 @@ fn test_search_paths_tracking_hash_different_order() {
             &opts.target_triple,
             &early_dcx,
             search_path,
+            false,
         ));
     };