correct platform parameter to OS/ARCH to fix image pull problem

[dustinblack]

Changes introduced with this PR

We have been seeing image pull failures with the podman deployer related to mult-arch images. It seems that we are passing the --platform parameter only the architecture value when this parameter requires OS/ARCH as input. The error reported by the workflow:

2023-11-02T17:31:51+01:00	debug	source=deployer	Pulling image: quay.io/arcalot/arcaflow-plugin-sysbench:0.6.0
2023-11-02T17:31:53+01:00	error	source=main	Invalid workflow (invalid workflow (failed to load schema for step sysbench_loop (invalid workflow (failed to load schema for step sysbench (failed to deploy plugin from image quay.io/arcalot/arcaflow-plugin-sysbench:0.6.0 (Trying to pull quay.io/arcalot/arcaflow-plugin-sysbench:0.6.0...
Error: choosing an image from manifest list docker://quay.io/arcalot/arcaflow-plugin-sysbench:0.6.0: no image found in image index for architecture amd64, variant "", OS amd64
))))))

Can be exactly reproduced with the command line:

$ podman pull --platform amd64 quay.io/arcalot/arcaflow-plugin-sysbench:latest
Trying to pull quay.io/arcalot/arcaflow-plugin-sysbench:latest...
Error: choosing an image from manifest list docker://quay.io/arcalot/arcaflow-plugin-sysbench:latest: no image found in image index for architecture amd64, variant "", OS amd64

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