Consistently use local helper image in CI

What does this MR do?

Uses docker load and docker import to load the prebuilt helper image.

Why was this MR needed?

Runner natively supports this functionality, which we've previously relied upon.

However:

• Some test configurations would pull in a helper image anyway, and depending on order, we might end with that one.
• Windows loads from an zstd docker archive image, which is only working on modern versions of Docker engine, and our Windows runners are using a version that is too old.

These discrepancies become apparent if you make a change where every test relies upon a needed helper image modification.

What's the best way to test this MR?

Pipeline should pass. There should be no log of "Loading helper image".

What are the relevant issue numbers?

Closes #38592 (closed)