Bump actions/cache from 2.1.4 to 2.1.6

Bumps [actions/cache](https://github.com/actions/cache) from 2.1.4 to 2.1.6.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2.1.4...v2.1.6)

Signed-off-by: dependabot[bot] <support@github.com>
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