Update actions/cache action to v2.1.6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
actions/cache	action	patch	v2.1.4 -> v2.1.6

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Release Notes

actions/cache

v2.1.6

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• Catch unhandled "bad file descriptor" errors that sometimes occurs when the cache server returns non-successful response (Catch and log unhandled exceptions stemming from closed file descriptor actions/cache#596)

v2.1.5

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• Fix permissions error seen when extracting caches with GNU tar that were previously created using BSD tar (macos: gtar: permission denied actions/cache#527)

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