fix(image-multiarch): skip qemu if only one platform is requested

[pmlanger]

https://typeform.atlassian.net/browse/PLT-000

Skip QEMU if only one platform is requested.

Mostly, we build ARM images on ARM runners now. There a few ML services that still build for ARM and AMD, they would continue working as today after this change.
Only thing it doesn't cover is something like this which are easy to fix (switch to ARM base image if exists, or use AMD runner) and slightly strange usage.

[gitstream-cm]

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