[google] suppress file_cache warning

[lwis]

Description:

googleapis/google-api-python-client#299 describes issue and suggested resolution.

Related issue (if applicable): fixes #5904 (issue raised in PR)

Checklist:

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass

[mention-bot]

@lwis, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mnestor and @bachp to be potential reviewers.