Lack of cache increases Kokoro flakiness

This is a grab-bag for any time Kokoro fails during downloading resources or similar. We cache dependency downloads on our other CIs, so such problems should be more rare. This issue should only be for flakes where caches on Travis CI or Jenkins would have been able to circumvent the failure.

Start by reviewing the Kokoro configuration and the existing dependency-cache setups on Travis CI and Jenkins. Identify the download-related flakes this issue covers; done means Kokoro has comparable caching and those failures are reduced or avoided.

Scritto dal modello di indicizzazione a partire dal testo della issue.