feat: honor BUILDKIT_PROGRESS as like Docker

[kzys]

BUILDKIT_PROGRESS=plain is especially useful for debugging. Before this change, it only works when DOCKER_BUILDKIT=false.

Change Summary

What and Why:

How:

Related to:

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Documentation

• [ ] Fresh Produce
• [ ] In superfly/docs, or asked for help from docs team
• [ ] n/a