Update the image in cloudbuild.yaml to support latest bazel

[douglas-reid]

The istio builder image (gcr.io/istio-io/istio-builder:0.1) has an old (no longer working with our codebase) version of bazel installed.

We should get a new image with the proper version of bazel, or revert back to gcr.io/cloud-builders/bazel.

[sebastienvas]

What is this thing for ? How is it used today ? you can use gcr.io/istio-io/istio-builder:0.3.2. While you re at it, could you also update https://github.com/istio/mixer/blob/master/bin/build_mixer_release.yaml#L5 ?