Only create k3s-images.txt on amd64

The list is the same across architectures, and is validated against the
list in git as part of CI... so there's no reason to be pushing it from
every pipeline. It's also causing conflicts when multiple pipelines try
to upload it at the same time.

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

scripts/package-airgap

@@ -9,4 +9,6 @@ airgap_image_file='scripts/airgap/image-list.txt'
 images=$(cat "${airgap_image_file}")
 xargs -n1 docker pull <<< "${images}"
 docker save ${images} -o dist/artifacts/k3s-airgap-images-${ARCH}.tar
-cp "${airgap_image_file}" dist/artifacts/k3s-images.txt
+if [ ${ARCH} = amd64 ]; then
+  cp "${airgap_image_file}" dist/artifacts/k3s-images.txt
+fi