Merge KUBEVIRT_TAG & KUBEVIRT_VERSION within our automation scripts

[lyarwood]

What happened:

$subject, this is trivial but we are pinning to a specific KubeVirt release using two different env vars at the moment. Merge these and use the existing latest_version logic

$ git grep -E '(KUBEVIRT_TAG|KUBEVIRTCI_TAG|KUBEVIRT_VERSION)'
automation/common/deploy-kubevirt-and-cdi.sh:oc apply -n $NAMESPACE -f "https://github.com/kubevirt/kubevirt/releases/download/${KUBEVIRT_VERSION}/kubevirt-operator.yaml"
automation/common/deploy-kubevirt-and-cdi.sh:oc apply -n $NAMESPACE -f "https://github.com/kubevirt/kubevirt/releases/download/${KUBEVIRT_VERSION}/kubevirt-cr.yaml"
automation/common/versions.sh:KUBEVIRT_VERSION=$(latest_version "kubevirt" "kubevirt")
hack/kubevirt.sh:export KUBEVIRT_TAG="${KUBEVIRT_TAG:-main}"
hack/kubevirt.sh:    git clone --depth 1 --branch "${KUBEVIRT_TAG}" https://github.com/kubevirt/kubevirt.git "${_base_dir}/_kubevirt"
hack/kubevirtci.sh:export KUBEVIRTCI_TAG=${KUBEVIRTCI_TAG:-$(curl -sfL https://storage.googleapis.com/kubevirt-prow/release/kubevirt/kubevirtci/latest)}
hack/kubevirtci.sh:    git clone --depth 1 --branch "${KUBEVIRTCI_TAG}" https://github.com/kubevirt/kubevirtci.git "${_cluster_up_dir}"

What you expected to happen:
A clear and concise description of what you expected to happen.

How to reproduce it (as minimally and precisely as possible):
Steps to reproduce the behavior.

Additional context:
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Environment:

• KubeVirt version (use virtctl version): N/A
• Kubernetes version (use kubectl version): N/A
• VM or VMI specifications: N/A
• Cloud provider or hardware configuration: N/A
• OS (e.g. from /etc/os-release): N/A
• Kernel (e.g. uname -a): N/A
• Install tools: N/A
• Others: N/A

[lyarwood]

/assign @lyarwood