fix(testing) retention for image-builds-with-cache artifacts

[alyssacgoins]

Description of your changes:
Add the current date to the cache key used in .github/workflows/image-builds-with-cache.yml. This forces the cache key to change daily, so that old image markers disappear after midnight, syncing cache expiration with single-day artifact retention.This change should fix the CI bug in which image artifacts fail to be located.

This PR is a follow-up to the cache lookups added in #12712.
Resolves #12734

Checklist:

• You have signed off your commits
• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.
• [] Make sure the changes work with DSPO. Run the tests in DSPO with DSPA images pointing to this PR's images

[google-oss-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[hbelmiro]

/lgtm
/approve

[google-oss-prow]

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