Add remote caching to bazel builds

[mkruskal-google]

This enables remote caching for all of our Bazel builds. This will have the largest impact on reruns, but should hopefully speed up all our CI builds.

Note: each job is given its own cache to avoid issues from non-hermetic builds. For example, if the version of python changes our current setup doesn't give Bazel enough knowledge to know the cache is stale. Ruby, PHP, C#, and Objective-c likely have this issue as well, since they all use non-hermetic genrules to grab system dependencies. Once all of these have been fully modified we can likely fall back to a single shared cache