Newly-added ZipFileArtifactServiceTest is flaky.

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ERROR: test_concurrent_requests (apache_beam.runners.portability.artifact_service_test.ZipFileArtifactServiceTest)
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Traceback (most recent call last):
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py37/build/srcs/sdks/python/apache_beam/runners/portability/artifact_service_test.py", line 215, in test_concurrent_requests
    _ = list(pool.map(check, range(100)))
  File "/usr/lib/python3.7/concurrent/futures/_base.py", line 586, in result_iterator
    yield fs.pop().result()
  File "/usr/lib/python3.7/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/lib/python3.7/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/usr/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py37/build/srcs/sdks/python/apache_beam/runners/portability/artifact_service_test.py", line 208, in check
    self._service, tokens[session(index)], name(index)))
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py37/build/srcs/sdks/python/apache_beam/runners/portability/artifact_service_test.py", line 73, in retrieve_artifact
    name=name)))
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py37/build/srcs/sdks/python/apache_beam/runners/portability/artifact_service_test.py", line 70, in <genexpr>
    return b''.join(chunk.data for chunk in retrieval_service.GetArtifact(
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/tox/py37/build/srcs/sdks/python/apache_beam/runners/portability/artifact_service.py", line 133, in GetArtifact
    chunk
 = fin.read(self._chunk_size)
  File "/usr/lib/python3.7/zipfile.py", line 899, in read
    data = self._read1(n)
  File "/usr/lib/python3.7/zipfile.py", line 989, in _read1
    self._update_crc(data)
  File "/usr/lib/python3.7/zipfile.py", line 917, in _update_crc
    raise BadZipFile("Bad CRC-32 for file %r" % self.name)
zipfile.BadZipFile: Bad CRC-32 for file '/3b2b55eb92de23535010b7ac80d553ec2d4bae872ac5606bc3042ce9313dff87/069587dcb8f8b63329ae53051ba79ba34ba0deb41c7a1e044280d7b6bb15e4f0'