ci: skip buildah unshare when the cache cleanup runs as root

scripts/test-linux-agent-build.py

@@ -174,7 +174,8 @@ esac
             for line in calls:
                 self.assertIn('||', line, f'{script}: {line.strip()}')
         cleanup = (ROOT / 'ci/cleanup-build-cache.sh').read_text()
-        self.assertIn('if ! buildah unshare python3', cleanup)
+        self.assertIn('if ! in_userns python3', cleanup)
+        self.assertIn('[[ "$(id -u)" == 0 ]]', cleanup)  # root 不进 buildah unshare，这台主机上它会报 memfd_create
 
     def test_linux_packages_preserve_installers_and_shared_library_payloads(self):
         with tempfile.TemporaryDirectory() as directory: