Mark the serve_stale system test as flaky

The serve_stale test has some inherent instabilities affecting many
different checks. While the failure rate isn't too high (about four
failures in past three weeks of nightlies), it gets ignored, because the
test has been unstable for a very long time.

(cherry picked from commit 1e0df480c7)

bin/tests/system/serve-stale/tests_sh_serve_stale.py

@@ -11,6 +11,8 @@
 
 import pytest
 
+import isctest.mark
+
 pytestmark = pytest.mark.extra_artifacts(
     [
         "dig.out.*",
@@ -24,5 +26,6 @@ pytestmark = pytest.mark.extra_artifacts(
 )
 
 
+@isctest.mark.flaky(max_runs=2)
 def test_serve_stale(run_tests_sh):
     run_tests_sh()