Automatic merge from submit-queue

e2e-runner: Get GCI builtin k8s version from GCS

**What this PR does / why we need it**:
The GCI release qualification tests use builtin k8s version to run tests. They used to get the builtin version by parsing descriptions of the test images, but that's unreliable because the description format could change. This PR makes it to read a file checked in GCS that has the exact k8s version number.

@adityakali @spxtr Can you review?

cc/ @kubernetes/goog-image