oc-mirror is unable to mirror the images when multiple channel streams are specified.

Issue

• oc-mirror fails to complete when multiple channels for release-images are mentioned in the ImageSet configuration.

• One such example of ImageSet configuration is as follows :-

kind: ImageSetConfiguration
apiVersion: mirror.openshift.io/v1alpha2
storageConfig:
  registry:
    imageURL: mirror.example.com
    skipTLS: false
mirror:
  platform:
    channels:
      - name: stable-4.15
        minVersion: '4.15.18'
        shortestPath: true
      - name: stable-4.14
        minVersion: '4.14.15'
        shortestPath: true
      - name: eus-4.14
        minVersion: '4.14.9'
        shortestPath: true
      - name: eus-4.12
        minVersion: '4.12.45'
        shortestPath: true
    graph: true
  helm: {}

• Upon initiating the mirror, the process fails with the following error :-

[root@localhost ]# ./oc-mirror --config=<config_file_path> mirror.example.com 
Checking push permissions for mirror.example.com
Creating directory: oc-mirror-workspace/src/publish
Creating directory: oc-mirror-workspace/src/v2
Creating directory: oc-mirror-workspace/src/charts
Creating directory: oc-mirror-workspace/src/release-signatures
No metadata detected, creating new workspace
error: error calculating cross channel upgrades: failed to get upgrade graph: NoVersionsFound: no cluster versions found for "amd64" in the "eus-4.18" channel