tests: mark test_extend_branch as flaky (#4718)

tests/func/experiments/test_experiments.py

@@ -1,6 +1,7 @@
 from textwrap import dedent
 
 import pytest
+from flaky.flaky_decorator import flaky
 from funcy import first
 
 from dvc.utils.serialize import PythonFileCorruptedError
@@ -205,6 +206,8 @@ def test_update_py_params(tmp_dir, scm, dvc):
         dvc.experiments.run(stage.addressing, params=["params.py:INT=2a"])
 
 
+# See: https://github.com/iterative/dvc/issues/4717
+@flaky(max_runs=3, min_passes=1)
 def test_extend_branch(tmp_dir, scm, dvc):
     tmp_dir.gen("copy.py", COPY_SCRIPT)
     tmp_dir.gen("params.yaml", "foo: 1")