fix(image-multiarch): skip qemu if only one platform is requested (#198)

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](https://github.com/Typeform/intigreton/blob/bcda0e16a73b98624c23bb3696f51005d3d466e4/.github/workflows/ci.yml#L66)
which are easy to fix (switch to ARM base image if exists, or use AMD
runner) and slightly strange usage.

Author: pmlanger

.github/workflows/image-multiarch.yaml

@@ -61,6 +61,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v5
       - name: Set up QEMU
+        if: ${{ contains(inputs.platforms, ',') }}
         uses: docker/setup-qemu-action@v3
         with:
           image: 567716553783.dkr.ecr.us-east-1.amazonaws.com/docker-hub/tonistiigi/binfmt:latest