fix: preserve .yarnrc.yml during yarn cleanup

.yarnrc.yml configures Yarn 4.x mode; removing it breaks yarn
detection and prevents yarn 4.x reinstallation on retry.

npm_install.py

@@ -2028,10 +2028,9 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
 
     def _cleanup_yarn_artifacts():
         """Remove corrupted yarn artifacts."""
+        # Do NOT remove .yarnrc.yml - it configures Yarn 4.x mode
         cleanup_paths = [
             Path("/root/.yarn"),
-            Path("/root/.yarnrc.yml"),
-            Path("/root/.yarnrc"),
             Path(os.path.expanduser("~/.config/yarn")),
         ]