- What I did
We already lock the index before writing to it. However, the write is not atomic; the underlying ggcr library writes it in place. That means that some other thread or process reading it can get a corrupt file if it is mid-write. It would be nice if we could make the writes atomic, but ggcr does not support it.

Instead, we use the same lock everywhere that we read the cache root index. That turns out to be only in three places, so I added a function cache.Provider.Index() which gets the root index, and that uses the lock.

- How I did it
As described above.

- How to verify it
CI

- Description for the changelog

lock when reading cache root index.