Allow updating an existing cache

GitHub actions supports caching across builds which is super powerful. However, once a cache has been saved it can no longer be modified. While this is fine for most scenarios where you need the cache there are a few times when you would like to update it.

I created this PR on the actual cache action to allow updating a cache when providing it in configuration:

https://github.com/actions/cache/pull/353

I created it after reading the related issue:

https://github.com/actions/cache/issues/342

However, it seems like something either needs to change in this package or server-side on the actions cache.

My use-case for wanting to update a cache is when using GitHub actions in golang projects. Go supports a test cache which allows tests to be skipped if the code they use has not changed, which can significantly improve test times (especially in large projects). Each time I run my tests, I'd like to maintain a single cache for tests, that way, when a run completes I can update the cache and all my workflows share a single test cache

There are some extra use-cases described on the issue above.

Start by reading the linked actions/cache pull request #353 and issue #342, then determine whether the requested cache update behavior belongs in this toolkit or the Actions cache service. The Go test-cache use case describes the desired outcome, but the issue does not identify files, tests, or a repository-side implementation boundary.