Verify gRPC cache downloads after flushing the output.

[benjaminp]

Make sure any internal buffers (especially compression) are flushed before attempting verify the digest of the downloaded file.

[brentleyjones]

Does this fix any known bug? Is it worth cherry-picking into 5.3?

[coeuvre]

I am not aware of any bugs this fixes. but should be easy to cherry-pick into 5.3 if time allows.

[brentleyjones]

@bazel-io flag