Replace cached state from the same commit, rather than failing the job

[teor2345]

Motivation

If a cached state job is run twice on the same commit on the same branch, the job fails.

If other required jobs depend on that cached state job, they are skipped, and the PR won't merge.

The resource 'projects/zealous-zebra/global/images/lwd-cache-4765-merge-59532a1-v25-mainnet-tip' already exists
Error: Process completed with exit code 1.

https://github.com/ZcashFoundation/zebra/runs/7588987700?check_suite_focus=true#step:8:72

Suggested Solution

Replace the image, rather than failing the job.

If that's not possible, it's also ok to skip creating the image. But the job still has to run successfully. That's not ideal, because it leaves an older (and possibly buggy) cached state image behind.