incorrect format for mappting when use mirror2disk with dry-run

when use mirror2disk with dry-run the ouput mapping is incorrect. 

always 

1. with imagesetconfig: 
cat config-addimage.yaml
apiVersion: mirror.openshift.io/v1alpha2
kind: ImageSetConfiguration
mirror:
  additionalImages:
   - name: quay.io/openshifttest/bench-army-knife@sha256:078db36d45ce0ece589e58e8de97ac1188695ac155bc668345558a8dd77059f6
2. run the mirror2disk with dry-run :
`oc-mirror --config config-addimage.yaml file:// --dry-run`

2. check the mapping , the format if incorrect , should with "file://"
cat oc-mirror-workspace/mapping.txt
quay.io/openshifttest/bench-army-knife@sha256:078db36d45ce0ece589e58e8de97ac1188695ac155bc668345558a8dd77059f6=openshifttest/bench-army-knife:latest

correct mapping should be: 
cat oc-mirror-workspace/mapping.txt 
quay.io/openshifttest/bench-army-knife@sha256:078db36d45ce0ece589e58e8de97ac1188695ac155bc668345558a8dd77059f6=file://openshifttest/bench-army-knife:latest