cosa run should't expect an image when --netboot is present

[jbtrystram]

see #3806

$ cosa run -c --netboot pxe/shimx64.efi -m 4096 --qemu-firmware uefi-secure
COREOS_ASSEMBLER_ADD_CERTS=y
+cosa:16> podman run --rm -ti --security-opt 'label=disable' --privileged '--uidmap=1000:0:1' '--uidmap=0:1:1000' --uidmap 1001:1001:64536 -v /var/home/tim/projects/rhcos.images:/srv/ --device /dev/kvm --device /dev/fuse --tmpfs /tmp -v /var/tmp:/var/tmp '--volume=/etc/pki/ca-trust:/etc/pki/ca-trust:ro' quay.io/coreos-assembler/coreos-assembler:latest run -c --netboot pxe/shimx64.efi -m 4096 --qemu-firmware uefi-secure
Error: No disk image provided
2024-05-16T15:41:40Z cli: No disk image provided