Fix QA image name in FOSS pipelines

What does this MR do and why?

In !98685 (merged) we started building QA images in FOSS pipelines also. However, we missed the fact that QA image name is hardcoded in the scripts.

This MR moves the QA builds to a standalone script, using scripts/build_assets_image as a reference, and thus fixes the image name.

Related to #375330 (closed)

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