[QE] Build test images on version cut merges to main and push them to quay.io

[jsliacan]

Follow-up to Issue #3933 .

Conditions:

• push to main
• commit message contains cut v... text, e.g. cut v2.30.0.

If satisfied, build test images like in #3933 , but do not upload them as artifacts. Instead, push them to quay.io to be consumed by downstream CI during release testing (which always follows a cut).

This step is currently manual, would be nice to automate it.