Automatic merge from submit-queue (batch tested with PRs 62495, 63003, 62829, 62151, 62002). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add a GCS mirror to WORKSPACE URLs, along with a script to update them.

**What this PR does / why we need it**:
We get occasional flakes when WORKSPACE downloads fail. This should make that much more unlikely, by having two independent endpoints to try.

**Release note**:
```release-note
NONE
```