Distroless version of Mariner's dependency on non-distroless version for testing

A testing issue was identified in the case where a build is queued to only target the distroless version of Mariner. The images build fine but the testing will fail. The tests expect that the non-distroless version of Mariner also exists in order for it to run some operations with that. Because it doesn't exist in the staging repo, you'll get this error from the test:

```
Standard Error: manifest for dotnetdocker.azurecr.io/build-staging/1767152/dotnet/runtime:6.0-cbl-mariner1.0-amd64 not found: manifest unknown: manifest tagged by "6.0-cbl-mariner1.0-amd64" is not found
```

This dependency needs to be accounted for so that the required image will be available for the tests.